Increase in oxidation peak after each run ( or even in each subsequent cycle) is the result of occurrence of some irreversible oxidation process in your system. For that, u will need to check ur system and the process that corresponds the said peak, and hence forth the product, which the process is not able to reduce. This irreversible change could be electrochemical, chemical, photo or induced by some other factor, which also needs to be found out.

 I could probably help but you did not provide details. Check my publications in my profile. Ask specific questions. 

Thank you all for your valuable advice. 

I sent you a copy of the paper which is more related to your question