thats cuz they pulse more when they're not in current. My guess is because current just blows them around so you cant see the pulsing even if it is (or it just makes it so they cant pulls...like it overpowers them?)
Some of mine are in very strong current, and they're takin off, but they dont pulse, they just wave around..
Others are in kinda "dead" spots, and they go crazy...then i've got some in lower flow (ya i've got lots of xenia) and they dont really do anything.