When we have a mystery dance, sometimes the person's name is clickable and sometimes is not, like a plain word. How come, why?

Are you perhaps mixing up the Mystery dance page (which has a link to the user you dance with) and the accept a regular dance (which only has the user name and no link)?

No, no, the mystery dance, sometimes it says you danced with xyz, I try clicking on that username and it has no clicking option.

Possibly the unclickable users are people who have blocked you, or ones you have blocked? This page could help you confirm that theory.

I thought so too, thanks, but there are usernames I don't know at all, new, or on no lists...