This depends on the equipment you are using and the concentrations. If possible with your setup you might want to try suppressing the background noise with a sulfuric acid solution.

I agree with Godts suggestion. Try by cleaning the column and Increasing your eluent concentration. Check each part for same flow rate. 

Thank you for your comments. I am using an IC from Shimadzu and it is still new. I am going to leave it under eluent wash overnight then see. Many thanks

Are you using a suppressor?  If so, perhaps the suppressor needs to be regenerated.

Yes, I use a suppressor. the suppressor still new and the instrument is still under method development and validation. I am using 20 micro Liter loop, does this affect the peak resolution and shape? This loop is the recommended one for such analysis. Thank you all

Even though the suppressor is new, it's possible that it might be in need of regeneration. Is the suppressor you are using a continuously regenerated suppressor or a "packed bed" type suppressor? If it's a continuously regenerated electrolytic suppressor, perhaps you should try increasing the current by 50% to see if this increases the peak area.

You didn't mention what column dimension you are using. I assume you're using a 4 mm column. The column dimension also influences your concentration sensitivity associated with a specific injection volume. 20 µL is reasonable for an analysis requiring low part per billion detection limits if you are using a 4 mm column. If you need single digit part per billion or even lower detection limits, you will need to use a larger injection loop.

I agree with most of Paul' comments. Column dimensions, suppressor regeneration etc. are important. To give you further support, you shoud give us more details:

Are you applying the same condictions as used in the columns test chromatogram?

Peak hight, background conductivity, concentration of the injected solution etc.

Is the separation and are the retention times correct?

Which concentration range you are looking for.

Check also back with your sales person.

Thank you all :) you gave me valuable notes. In fact, I passed this problem after regenerating the suppressor beside increasing the injection volume from 1 ml to 1.5 ml. I think the injected volume was not enough under not regenerated suppressor.

Many thanks