I had the same problem with my seat belt when I bought my jeep, I took mine apart and inside there is a peice of tempered steel that is like a V shape that puts pressure on the button that you press to release the seatbelt. Over time that looses its rigidity or it breaks. Best bet is to replace the seatbelt. Not worth fixing when its ment to save your life in the event of an accident. Check the local pay and pull you should be able to swap in another belt from a different model jeep.
87 YJ 4.2L, 3 Tops, Full hard and soft doors and hard half doors, Nuttered, Weber 32/36, Flowmaster 40, 4inch RC, BrandonYJ 1inch Booms, 33x12.5 Baja Claws, Posi-loc, Round tail lights, 95 Mitsubishi Eclipse seats, Grant racing wheel, Homebrew front stinger, 4 fogs on bumper, 2 on windshield, Built on family rollcage, Xplod deck and 2 12's, 6x9's in homebrew soundbar.