Whoops, sounds like that worked too well in the other direction. You are certain that these sims (married or not) know each other very well, is their relationship score very high, and are they actually attracted to each other? Does Scan Room on the active sim with the target in the same room result in attraction/interest scores well above 100? Are their needs taken care of? In my experience, with my settings anyway, sims who are tired, hungry, low on hygiene, have full bladders, not having fun, aren't going to be interested in physical displays of romance. Nor are sims who just woohoo'ed one or more times, there's a cooldown penalty period in interest scores involved, but sounds like that isn't relevant here.
I would concentrate on Woohoo first and TryForBaby later to see what's getting in the way there.
Instead of going the Romantic interaction route, does it work out any differently if they start cuddling on a bed, move onto kissing and making out, then woohoo from that sequence? Or, if you have Generations, can they woohoo in a shower if you click on the object instead of the sims after working them up to Extremely Irresistible? Sometimes sims need some pushes in the right direction if they haven't been physical with their partners on-screen yet.