Your problem is that you have radius culling enabled (the R with the red circle around it in the tools panel, this thing:
).
When enabled it hides all brushes outside of the radius, which is centered on the camera in the 3D viewport. This means when you compile your map the brushes hidden by radius culling will be ignored and will not show up in game.
The quick way to enable/disable it is to press c on your keyboard.