So I have the tower built, base colours in, rocks nicely hand painted.
Trying to paint the Texture on the base, nothing happens. Tried other parts of tower, same problem, no painting shows.
Created a new model (cube) to test my method is correct and texture mask works fine.
Back to the tower, when I Resize the brush, I can see the texture mask:
Blend is Mix, Strength is 0.5
Checked Normals, they are all blue (ok).
Stuck! Would love suggestions.
Full screenshot:
Try appending in the objects to a new file. That gets around some odd unknown glitches.
I had this problem recently, here are a couple of things I make sure to do before I texture paint:
First, make sure that the object you are painting is selected in object mode (before going to texture paint mode).
Second, make sure that the image texture in the shader editor is selected.
Sometimes switching to the texture paint workspace will deselect everything even though the object you want to paint will be in the viewport. If these suggestions haven’t helped, I’ll be glad to think of more solutions. 
