For starters, change all of your walls to solid by selecting them and hitting F. Not having much solid in your map makes it an issue.
Here are some pertinent threads brought up by the search:
http://forum.cubers.net/thread-1298-post...l#pid20517
http://forum.cubers.net/thread-1388.html
http://forum.cubers.net/thread-2032.html
Here are some pertinent threads brought up by the search:
http://forum.cubers.net/thread-1298-post...l#pid20517
http://forum.cubers.net/thread-1388.html
http://forum.cubers.net/thread-2032.html