OK, so it's not the usual issue that I am aware of. Very strange. Are you able to get a simple level change working between a couple of example maps, like say, just a short corridor? Or does this only affect a specific couple of maps that you are working on?
If you can get a simple levelchange example map to work, then it might be an issue with the setup in the map you're working on and you might need to upload them to the problem maps vault so that we can see the setup, or post screenshots of your map grid and the entities used to changelevel.
But first things first, can you get a transition to work between two simple example maps?