I am trying to do titration based on the UV-Visible spectrometry.
You may have several causes. You might get foreign light if the equipment is not too new, or your compound might decompose on measurement, your sample's concentration might not be right for that wavelength, look up literature analogies, and try to reproduce them. That usually works.
Any light scattering in the sample cell could reduce the amount of light getting through to the detector in the spectrometer. Could there be some very small particles in the sample cell? If colloidal, they could be very difficult to see with the naked eye. Particle formation might occur from decomposition or reaction of the analyte or the presence of some impurity in the sample. Try shining a bright beam of light through the sample cell to look for a Tyndall effect.
check the lamp power, check the zero line before the measurement starts (no cuvettes on the both paths of light ) ,check the blank to be compatible with the solvent and measure absorption relatively to the blank, check the time-stability of the molecule you intend to measure, check the concentration of the active component- that is in correlation with the efficiency of absorption on the relevant/desired WL
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