This might be a stupid suggestion, but I have had habits and techniques I just never questioned – if this is a regular problem and not just some few items (some zippers are just shitty, I get that), have you tried changing your technique? Like pulling the end of the zipper down until it’s under a bit of tension with one hand and then pulling the actual zipper thingy away from the flap while closing the zipper with the other hand. That usually works for me.