Try changing your pump parts, particularly your valves since they are notorious for altering pump suction with just a hairline crack in them. How old is your LO, and how do you know that your supply dropped?
She's 9.5 months now. I have been pumping since I returned to work when she was 4 months old. Supply has been dropping steadily from 200ml per pump to only 100-130ml now.
Esther Lim