My engaged sims can't woohoo or make out

As it says.  A sims couple of mine have been together for a little while and whenever I attempt to have them make out, woohoo, or try for a baby (anywhere, not just in beds), they act like they've been reset (their arms go slightly out and they straighten up).  They do get a positive relationship gain and no other couple in my file has this problem (that I know of)