Sims can’t woohoo

I’m not sure if this is a technical issue or a bug but my sim has just moved her newly aged boyfriend into the household and they can’t woohoo or try for baby at all.  They discuss it and do a little dance but then the action just disappears from their queues.  My mods are up to date, there is plenty of room around the woohoo options, and I even took them to the gym to see if they’d woohoo in the shower there but nothing happened.  This has only happened today; when I played last Sunday while the heir was still a teen, she and the boyfriend were able to mess around in the closet and shower. 

Could it be something to do with aging the boyfriend up then moving him in?  Or the fact that they’ve been dating a while?  It’s the last generation of a legacy challenge and I need them to have babies without resorting to having science babies!