Clients don't send a zap receipt, for some reason the wallet isn't sending your zap receipts, idk why.

Might be a good exercise to check kind:9735 with P and p tags for sender/receiver just to be sure.

 It might also be client issues where the wallets aren't publishing to your relays, because the zap request has the wrong relays in the request, have you tried using another client?  

 Yes primal and Amethyst stopped working randomly one day.