You changelevel entity setup is probably set up incorrectly.
The most important part of a changelevel is the info_landmark entity, which is needed in both maps.
Here's an image to help you get it right:
Note that the distance between the info_landmark entity and the trigger_changelevel brush entity must be the same in both maps! One unit off the changelevel will fail, resulting in the player ending up in the void, or worse: a game crash.
If you're still having problems getting it to work, upload the map to the problems maps vault so we can have a look. Or, if you don't want anyone to see your map, send it to me via email.