The only time that that's happened to me (even with an almost full relationship bar) is when my sim was trying to flirt with another sim who was already married. I think in your case though, the sim might be Unflirty. I find that when I can't flirt with a sim, for whatever reason, it's best to try to raise their happiness meter. Is the other sim possibly hungry or tired? It might just be that they are in a bad mood.