sounds loke you temp could be off I bought a cheap 30.00 heat gun at home depot to check all my stuff. You may need to try that I think Harbor freight has them go on sale real cheap. I have never had a problem with jpss paper it is pretty forgiving. Peel real quick after you lift you have the correct temp and time...
I would take the same shirt you messed up make a small print and try lowering the temp maybe 20 degree at a time until it comes out go. Sounds like you already ruined the shirt so nothing to loose