Looks like you are painting too heavy mate.. theres a LOT of paint there.
Are you using a brush or spraying. Either way what you need to do is apply the frisket and make one VERY light coat to seal the edges... let this go off and then apply the top coat. Spray enamel is a cow to use as its very thin and easily leeches under your mask. You don't need to build up heavy paint layers.. less is more with this and you barely need enough to just get an even colour.
A nice trick is to use something like a matte white undercoat first to make a very light spray.. this seals the edges of the mask and prevents leeching... you can then go ahead and use the spray enamel. Youll find it easier to cover a misty white colour as opposed to a black or red..... but I cannot emphasise enough that you need to go very lightly!!
+1
6 light coats are better than 2 heavy coats. There is a temptation to "get the job done", but as Gavin pointed out - go slowly and once you have enough color - STOP.
