What's the done thing when you can't kiss the person above you in a 'Kiss The User Above You' thread?

potentially a stupid question, but why would you not be able to kiss them?

If they've blocked you, probably. If you've blocked them, you just wouldn't see their post at all so they wouldn't get a kiss from you then either.

I'd probably just go for the person above them instead, or maybe below me if their post loaded when mine did/kiss that person back ¯(ツ)/¯

Ahhh, I see. I'm guessing they probably wouldn't want a kiss from you if they've blocked you anyways lol - so yeh probably best to just go above

Kissing threads are post first then kiss, so by participating you are taking your chances anyway because failing to kiss the user above AFAIK is not breaking the Rules. Users cooperate and do this anyways because they want to and it works. If they've blocked you kiss above them, and then, do an Event-based kissback on the person below you. That way neither user misses out on your sweet sweet lovin'.

Or kiss neither, and do the billboard.