Why is the jack on my headphones failing

The Jack on my headphones is failing for reasons I can't quite comprehend. Headphones worked great and then one side started cutting out, I opened them up and the jack is encased in plastic and you can't really see it's insides. Any way I thought it would be a bad solder joint on the jack but when I remove the PCB from the casing and push the jack farther in it works fine. when I put it back in the casing it won't work. The plastic casing doesn't allow the jack to go in as far.