When I'm painting I use a heat gun, much hotter than a hair dryer to the point I almost can't hold the shell as it gets it so hot. I let this cool slightly before putting on the next coat.
I also try and cut along the mask before I move it to save this. Only had this problem with masking film and water based paint.
You could try keying the shell up too, that would make the paint stick more but I'd say the best idea would be to cut along your design.
Also keep the painted area as confined as you can, spray everything to make sure you get an even coat will load the masking up you haven't removed so then by the time you're on your last colour it's all a bit thick and therefore might lift ???
Trial and error is usually the key mate, none of this is ever easy
