Assuming that your pump is working properly, there might be either a leak or something is outgassing within your chamber. Leak testing using a mass spectrometer should help to identify the problem. You may google possible reasons for leaks and how to detect them, see for example

https://www.lesker.com/newweb/technical_info/vacuumtech/leakdetect_01_overview.cfm

if material to be deposited is not degassing in the chamber than possibility of leakage from any of the window is there. such leakage can be detected with He-leak detector system. so try to find the system , connect the chamber with system and create vacuum then shower probable places of leakage with He-gas, detector will buzz on sniffing the gas.

Some times we bake the chamber for degassing.

There are many things that can affect your base pressure.

As others have mentioned the first likely culprit is gunk in your chamber. If you have a small quantity of material with a vapor pressure of 3e-5 Torr, your chamber will stop at 3e-5 Torr until all of the gunk has evaporated. How long does that take? Say you have an effective pumping speed of 100 l/s (the speed of the pump depends on what the gunk is and the pressure. It also depends on and may be dominated by the conductance of the path connecting the pump to the chamber) At 3e-5 Torr, a 100 l/s pump removes 0.003 Torr-l/s. You know that the volume of 1 mole of an ideal gas at 760 Torr and standard temperature is 22.4 l, so you are pumping approximately 2e-7 moles per second. You can see that even a tiny amount of gunk (finger-print!) with an unfortunate vapor pressure (oil) can hold you chamber at a fixed pressure for a very long time. If the vapor pressure was significantly lower, it wouldn't hold your pressure up. If the vapor pressure were significantly higher, the material would pump away in a reasonable amount of time. These moderately low pressures are where the troublesome stuff resides. Even water can be a problem. The vapor pressure of water is much higher, but that is for a puddle of water. A thin coating of water on the chamber walls has a much lower vapor pressure particularly if the walls are coated in some hygroscopic material, and opening a chamber to humid air can result in a slow pump-down.

The best solution to this problem is to be extremely careful to never get any gunk in your chamber. (you might want to tear down and clean) However it is impossible to keep out all gunk. Just try to minimize the quantity. In ultra-high vacuum chambers we will often keep the chamber just barely tolerably hot when opening and changing things just to keep down the amount of water vapor that adsorbs.

The next best solution is to raise the vapor pressure of the gunk. This is done by raising the temperature of the whole chamber and any gunk contained within. For example wrap the chamber in heater tape and cover that in aluminum foil. You can drive the heater tape with a variac, You will want to monitor and control the temperature. The limit is determined by whatever components are in the chamber. Often Viton seals set the limit. Anyhow, raise the temperature to whatever the components allow, (100C?) This greatly increases the vapor pressure of the gunk and the removal rate increases accordingly. The removal rate depends on the vapor pressure of the gunk at the applied temperature and the pump speed at the resulting higher pressure. In turn, the bakeout time depends on the removal rate and how much gunk you need to remove. (hopefully a fly hasn't entered your chamber!) These things can vary a lot, but typically a day is a reasonable bakeout.

The next likely culprit is a leak. The leak detection link given by Mr. Mitterer should tell you all you want to know about that. Assuming a reasonable pump speed, 3e-5 Torr is a big leak and should be easy to detect. Leaks usually happen at seals and the most likely seal is the last one you opened and changed. If you don't have a leak checker, you might want to just open and remake the last flange you touched and see if that fixes it. This may sound crazy, but for a big leak you can sometimes find it with a squirt bottle of methanol or alcohol or something else you don't mind briefly introducing to your chamber. Squirt likely places, and when the alcohol hits the leak the pressure will fluctuate. Generally the liquid momentarily blocks the leak and the pressure drops.

Gunk or a leak increase the load. The other possibility is that the pumping is reduced. People often ignore the conductance of how the pump is connected to the chamber, but that can often be the limiting factor in the effective pump speed. If you have changed anything in the path from the chamber to the pump, you might want to calculate the conductance and make sure you haven't limited yourself. However, that is generally more apparent in the speed of pumping down rather than the base pressure, and I assume if you had changed anything coincident with your problem you would have already put it back.

Turbo pumps can develop problems and essentially lose speed. This is a more rare problem, but not impossible. Does the turbo-pump make the same pitch sound as it used to? A more likely candidate for a problem is the backing pump. Mechanical pumps require maintenance. If this problem has creeped up over time, you might not be getting as low a backing pressure as you used to. If that is the case you need to tear-down, clean, and put fresh oil in the mechanical pump.

That's all I can think of at this time. I hope that helps.

Thanks a lot for all your valuable suggestions i am trying to degas the chamber as most of you suggested. unfortunately i don't have the He Leak detection system. I just hope there is no leak in the chamber. I will update you with the improvement if i find anything new apart from this discussion. Dear @Mike Albert Special thanks for giving in depth views on the problem.

Dear all , the problem is solved by subsequent cleaning using acetone, IPA and baking the chamber using substrate heater at 800 degC for 8 hours three times. I thank you all for the discussion.

By substrate heater, do you mean you are only heating your sample? I feel sure you didn't heat the whole chamber to 800C. The problem with this is that it just moves the problem around. Sure, you evaporate the gunk off of the hot surfaces, but they travel directly to the cool walls where they adsorb and once again hold your pressure up. If you were successful with this technique it was probably more due to the cleaning, or possibly because your 800C sample made the whole chamber a little warm. You would get further faster if you intentionally heated the whole chamber to the level that the components allow.