Did you set the AI for them to try to use it a lot? Are you sure its set to custom AI? Try setting the top line of the AI to Self MP > 10% Cure Light. That way they'll try it near-constantly without dying of overcasting. Also make sure there's not something in a sandbag nearby.
You can quickly train multiple pets by setting their AI to have that top line and no attacks, dismissing your pets, getting in a shelter with a house board, casting summon monsters, isolating a putit with deep water, killing everything else, resummoning your pets, and waiting/resting (riding a bell or equipping fetter of earth if possible) or fishing. This should work for teaching any skill or spell that doesn't kill the monster (healing, buffs, debuffs, etc.).
Thanks!