A “fanwank flop” is when a really hyped or highly tipped Eurovision entry ends up doing really badly in the results, or generally its result fails to meet fans expectations.
Why do these songs flop when they appear to be very popular? Maybe they got lumped with a really bad place in the running order. Maybe the singer’s live performance wasn’t so good. Maybe they got the staging/costume all wrong. Or maybe their high ranking in the OGAE polls gave a false impression of how the general public would vote on the night. Who knows?