I got a few reasons why check the following:
1)Extract the pk3 file to C:\Program Files\LucasArts\Star Wars JK II Jedi Outcast\GameData\base
2)check it's a pk3 file
3)check you have the map itself from:
http://www.jk2files.com/file.info?ID=4534
4)check for bugs at your base folder.
And thats the lot any other reasons i havent mentioned
try them your self. If these reasons don't work then they just don't work!