I think this depends on the fish actually.. I have noticed that fish with thicker spinage in their fins it often only heals and never grows. I've noticed with koi.
Spines and rays are very specific terms. Was it the rays in the caudal fin that failed to grow back? Or was it spines in the dorsal or anal fins? I am also noticing obvious growth on one of my boss' "dragon show bettas" after his fins were shortened by platyfish. They only have one, tiny spine in their body...so all of the fins are growing back.