I get that occasionally too but couldn't say what's causing this. The thing is that there is so many different wallets and nostr clients and some support all kinds of zap receipt tags, others don't and some use older methods. There will be always issues in sending, receiving, routing, implementations etc etc.

At this point if I can't zap or someone tells me they couldn't zap me, I shrug and carry on. Yesterday for example coinos users couldn't even zap coinos users and nobody knows why. Today it's working again.