thank you very much for the information.
As I can see you are doing everything right.
I think it has something to do with the heat press. Maybe the temperature is not rising fast enough or the pressure is too low.
I would try it again by increasing the temperature to 375-390°F.
If you start to peel after rubbing with a cloth over it and you see some toner rests on the transfer paper, rub again for 1 or 2 seconds and continue peeling, Repeat this step untill it works fine without any toner on the transfer paper.