ZOMBIE THREAD. but I believe I have an answer, and did not see any more current threads.
I had sims that could spar in Shang Simla, but could not spar at home.
on the first visit to China, I had not picked up any equipment, so there was no way to raise the skill at home. On the 2nd trip, I bought equipment. One of the sims got a wish to spar, but couldn't. I tried all 3 sims, none of them could spar.
I sent two sims to train, and they both gained a level AT HOME.
Sims 1 and 2 could now initiate sparring with each other and with sim 3. Sim 3 still couldn't initiate sparring with either of the other two.
The second Sim 3 gained a level "at home", then he could initiate the sparring.
So, it seems like if you're having trouble getting the "spar" interaction to come up, have your sim gain a level of Martial Arts in the "world" in question.