I used to believe the story about returned fish spooking others. Then I deliberately did so and found it rarely ever happened, PROVIDED THAT THE FISH ARE QUICKLY RETURNED WITH THE MINIMUM OF HANDLING. This even applies to perch. Indeed, I've sometimes caught the same perch again a short while later!
OTOH, if you take a while in getting the hook out or weighing or photographing the fish, then it probably will spook other shoal members. Mind you, it might be that your catch rate falls off purely because you've stopped feeding whilst dealing with the fish!
There could be umpteen reasons for missing positive bites apart from it being small perch. One of the most likely is that the perch are feeling some resistance.
Try float fishing, or if ledgering make sure you use a heavy weight so it stays put and the indicator moves instead. If the weight moves the fish will feel it.
Also make sure your rod points down the line - if it's at an angle the perch will be able to feel the increasing resistance on the rod top. If none of these work try increasing the distance between the hook and weight/bottom shot.
Talking of hooks, one that's too small for the bait can cause missed bites, as can the hook twisting and the point being masked by the bait.
There's just a few for starters. I'll probably be able to add a few more tomorrow when I'm not so tired!