After a log of digging, signature verification, code debugging and reading primal server/caching source code, I'm 99% sure my "test account" used for zapping doesn't pass the "is_human" test and primal ignores them. (although they increase the stat counter, which doesn't do the is_human check).
Other zaps I miss from other people is probably the same problem, their npub does not pass the is_human check.