So are you directly spraying the food at the clam with the baster? If so, stop doing this, it is really bad for the clam. You will clog the clam's gills blasting a clam with food from a baster.
What kind of lighting do you have? I personally don't think you need to feed clams directly (ie any sort of special feeding for the clam besides good lighting). Before I took my tank down when I lived in LA, I had a 1.5" and 2.5" maxima that were not directly fed. I put in a few capfulls of DT's in the tank once or twice a week, but that was it. They were under 400w halides and were growing until I sold them.