Hi Felix (@fxrob). I have had similar problems at times, sometimes with Eduard masks, sometimes with others, and sometimes with masking tape that I cut. I think if you leave it on for too long, the paint fully cures and loses its elasticity that it has when it is still curing. The other issue is how much paint and other layers you have sprayed over the masks. The more paint you spray, the harder it is to get a clean separation. For me, there is usually a color layer, then a gloss layer for decals, then a flat layer, then some weathering, and then probably another flat layer. I guess it also depends on whether you are spraying acrylic, lacquers, or enamels. In any case, the only way I know to be sure you will get a clean separation is to lightly score the edge of the mask with an Exacto, then carefully pull the mask off. Good luck.