Why I can't paint with more then two textures in landscape painting?

I have an issue with landscape painting. when trying to paint more then two textures on the same part of the landscape it will stay as the default chequered texture and I do not know why is this happening. I can only paint with two textures when I try to paint with another one that part of the landscape will turn into that chequered texture even after the shaders are compiled.

I would like some help please.