ok, just make sure you have the following:
2 landmarks entities in each map with the same name.
Name: landmark1 (example)
2 trigger_changelevel brush entities in each map.
Name: tomap2
New map name: map2
Landmark name: landmark1
2 trigger_transition brush entities in each map.
Name: landmark1 (landmark's name)
Since you have a large map, make two small maps that are similar and connect them using a changelevel. Get that to work first then use the working test as a model for the change in your project map. If this still doesn't work, try the tutorial on it or maybe even download the example map to be used as a model.
I've had trouble with these before, and what usually causes it is the lack of the trigger_transition entity. If you get an error message, the main problem i got was getting stuck in the walls of the next map, I think it gets confused which entities to transport to the next map without the transition trigger. Always be sure you have everything labeled correctly as well.
http://twhl.co.za/tutorial.php?id=62 tutorial btw.