How come? Hard to answer this way. Could be so many things. As mohaa_rox asked, is the map big? Do you get any messages in mbuilder? Does it just stop compiling or does it seem to skip Rad and make a fullbright map? Plz take some more time to clear up the problem for us.
But, if there's nothing more to report and it just seems to hang, the map is probably very big or complex to the compiler. If the map is completely sealed off from the void (no leaks) try makeing a leak in the skybox to speed up the compile. Maybe this way you get to see what could be wrong.