Glass minnow trap

I use my glass trap to get bait and there is a local river I go to and trap them and I stop at a certain ramp and I cant get these silver sides to go in and there seems to be no pressure from other people there. Now if I go to one of the other ramps along the same river, and set the trap out there I can get lots and lots of river shiners (silver sides) on other ramps. Can anyone answer why one location is more productive than the other. All I use is crushed up crackers in the trap.