Paint lasting for a few frames

Hello everyone.  First of all, i'm a beginner in After Effects,so i really hope someone can help me. I'm

having some trouble with the paint brush tool. My idea is to make an animation project.  I started with watching a tutorial, i firstly had to mask and the paint out some parts of the character. The problem is that the layers where i painted on for some reason last for a few frames. Let me explain better: The first seconds the layers are as if i never applicated any paint on them,and at some point  the paint comes back. In short,it applicates just for some frames. I don't know if i was clear enough,but i hope that someone managed to understand the problem,if anyone can help, it would be lifesaving. Thanks for reading.