I have been using the poly storm jets for years now, and I love them!
If one is not locking in, do some process of elimination. Does that jet work in another spot in the spa? Does another jet lock into that specific spot? If the problem follows the jet, there are two little tabs on the body of the roto jet that lock it in, sometimes they break off. Replace the jet. If the problem stays with the fitting on the wall, it could have some plaster lodged in it, preventing the jet from going all the way in. It is also possible that the plaster around the wall fitting is too tall, and is preventing the jet from going all the way in. The plaster should be flush with the wall fitting.