First of all, has this only happened recently? if it has, the simple solution is to wait it out. Bungie constantly modyfies and fixes maps, and they rotate around all maps usually fo fix stuff. Sometimes a map isn't accesible for a couple days. If you have been keeping up with bungies weekly updates, (bungie.com) i'm sureyou'll c that the map is under consstruction or something similar. this is the third question i answered about loading maps on halo 3 today

. Bungie should really inform the public about these errors in the game, and not just overlook the problem by loading new maps. I hope i answered your question, bye