DSF

Just a thought. What are the t-shirts you are printing onto?
A customer brought in some polos from a well known chain. I used my normal vinyl that works on everything else but no way would it stick to these shirts. There was some sort of impregnation in the shirt that wouldnt allow the glue to stick. Maybe that is your problem.

Another possibly silly answer - anywhere near seams of the garment? Occassionally you get some not stick brilliantly near seams, as the seam stops the press applying as much pressure to the area. If this is the problem, just press as normal then move so seam is away from under press and press again.