Just tried again manually. My guess is you are self-hosting Alby Hub and there is some kind of networking or other issue which is messing with the zap receipt generation or sending to relays...
Discussion
Yeah I set up a self hosted Alby but at the time I was using Lifpay for Zaps. Lifpay pulled the plug so I flipped to my Alby instead.
I reckon I'll move to something else.