by Marder » Thu May 16, 13 9:07 pm
The usual issue is that the mapper didn't make the door in a clear space, not touching any floors or other brushes or walls. If that doesn't fix the problem and the tutorial isn't working, then usually it might be some problem even more basic than making a door, such as how to align something to grid, or work from grid properly.
Try making the door, in a clear space within your initial subtraction. Rebuild your map before you start, make sure it is aligned or snapped to grid and follow the process properly, it should work.