Verify that you have the apropriate blank at each condition, pay particular attention to solution turbidity in blank and test solutions

Use matching cuvettes for blank and test solutions

Finally, if you are measuring enzyme activity, the specific value of absorbace is often not so important as you have to work with the slope of the linear portion of the time course. I hope this helps.

please check the blank quality well. temperature and light stability during the work by spect. good luck

Have you checked the stability of your substrate at pH values you are exposing it to ?