I've seen the same, sometimes it works, sometimes it doesn't, even between two zaps to the same person. Sometimes a zap receipt shows up on nostter, but not on Amethyst.

But I guess a lot of relays puke on my follow list 😆 

 also sometimes people's zap notification is just broken, for example my cln-zapper-rs plugin was stuck for a while
when i noticed it and fixed it, quite a few outstanding zap notifications must have gone through 😅