I get this a lot. It sounds ridiculous, but is the sim your sim is trying to flirt with already in a conversation with someone else? I've had it before where romantic interactions don't work because the game hasn't recognised that a sim is no longer in a conversation with another sim that isn't mine, if you see what I mean. It often happens when you interrupt a conversation in order to get the targeted sim in a conversation with yours - the target sim still has the blue relationship pop-up icon occasionally, as if their relationship is still increasing with the sim they were talking to before.
I believe I've also heard the playing tag bug - it would make sense, because almost all the sims that have been affected by this in my games are either childish or teenagers who were friends as children. In both cases, I would suggest resetsim as your first move, as this would cancel all interactions. If this doesn't work could you go to Edit Town and place another version of the sim in the town? Or you could add the targeted sim to the household so that you're in control of both, and perform romantic interactions while the target sim is active?