Importing Scenery into Scenario

[Philly]

OK, here's the issue. Whenever I import a weapon into the scenario of a multiplayer map, and then swap a piece of scenery for it, it dissapears after about 30 sceonds of starting a game. It's really annoying me because I have to swap weapons out and sacrifice them, just to get new ones. Does anyone know how to fix this? Any help is much appreciated.

Phil