Why doesn't my cutsom-made TMJ splint fit anymore?!

When any appliance is not worn regularly, any change in the mouth will cause it to no longer fit. If you have primary TMJ (aka internal derangement), and you were being treated by a dentist who really knows how to treat the condition, then you should have been told to wear your splint 24/7 until permanent changes can be made to your teeth, bite, and jaw so that you no longer need to wear a splint (aka Phase 2 treatment). But if you have secondary TMJ, and only need to wear your appliance as needed, then your teeth have shifted to the point where the appliance will no longer fit. Do not try to force it back into place; you need to have it adjusted by a dentist. Most times adjusting the appliance will help but if the shift is major then a new appliance will be needed.

functional orthodontist; treated TMJ since 1984