What can be the reason of credit score drop if making payments on time?

There are a number of reasons for which your credit score may drop. if you think there shouldn't be any negative item in your reports, then check your credit reports. There might be an error, for which, your credit score is getting reduced. Also, check your credit utilization ratio to make sure you're not near the limit. Even then if you don't find the reason, talk to a financial adviser. You will surely be able to increase your score.

Other reasons why your credit score has dropped are:

Using more of your available credit limit
New hard inquiry on your credit report
Age of credit history shortened
Change in the credit scoring model