Even with needs fine, mood maxed and relationship maxed they kept flicking to flirty for a split second after romantic interaction, then going straight back to 'okay'. I've seen a lot of topics on a search, but most of them fairly old, and no fix. Apparently it hasn't been fixed with patches either. Fortunately I hadn't gotten very far with my game, so I'm starting over. I wasted a whole bunch of time before realising it was a glitch of some sort. He was trying to romance Madison VanWatson (not my choice really, just building up LTR points) - she's not unflirty or commitment issues. She is childish, but I've never seen that be an issue before.