Hey,

If you map is looking like this, and you can't seem to find a fix, read on:

Open your map, go to Textures > Usage > Tools:

About half way down the textures box, you'll see a Pink box called "Light Grid Volume", Click on it.

With the Light Grid Volume selected, draw a big box around the playable area of your map (The Places people will go) Make sure you get the hight aswell.

Now, save and compile like normal :)

Note, Don't put the grid where players don't go, its a waste of memory! Also, All credit goes to techno2sl and Rasta!