Landscape material problem

I created 4layers for my landscape,but when I paint an area with more than two layers the area turns gray like non material applied.

this is what the issue looks like and how I setup the layers .
hope anyone can help me about this.

Try adjusting the preview weight of you layers. It determines the order of the layers(stay between 0.0 and 1.0) A higher number will be painted over a lower number. The issue you have may becaused by the fact that you have 3 layers with the same preview weight value(namely 0.0).

Though I do not know if this is the cause of your issue.