Hey Gangster47.
From the looks of it, that rotated section is not on the grid, and is also slightly overlapping other brushes on it's edges.
Try setting your grid to 1, snap that brush to the grid with
ctrl + g and adjusting the vertices (if necessary) so that the wall section butts up against, but does not overlap the floor/ceiling/or other wall sections.
Make your wall corners look like this in Radiant (If you have ever done
deck construction/carpentry, you know exactly the way this works).
Overlap the walls and the adjust the corner vertices to the meeting points (green
spot) shows the central meeting point that will butt the wall with minimal change to the thickness of the rotated brush:
The bottom wall is the finished result.