Geometry problems

Can anyone explain why this is happening and how to fix it?

This is a normal BSP box turned into a wall and all of a sudden half of it is gone. Only at this precise part, if I move it else where it won’t happen. Also if I make this a static mesh it will solve the problem. But for the time being I need to modify and play around as a BSP but its annoying to work like this.

Anyone?

more than likely you have a second brush set to subtractive that’s hiding that part