maybe you have made an really big map/the map has really a lot of actors or something along these lines.
or it could be a problem with the map .bmp file(like being out of palette or something similar to this).
OR it can be a faulty .lua file for the scene messing around with cortex command somehow.
try to first
A)read this tutorial just to make sure than you know all there is when it comes to missions.
B)check the .bmp files to see if they are in palete/copy paste them in a preexisting map to make sure it is in palete.
C)use a simpler .lua file than you know it works/do not use a .lua file at all,if this works it means you have messed up with the file somehow.
also upload the mission if you want to let us see it

Well, my problem is, it worked the other day. I can reduce the actual size of the terrainmap itself, but I don't think that's the problem.

Other than that, it's got 1 actor (a brain), and no Lua.

It does however have a good number of vanilla bunker bits in it, which I suppose I could temporarily remove to test that route...

Regardless, thanks for the input. Will tinker.

i am pretty sure than bunker modules do not affect the game at all
so it should be something else going on...
probably...
what actor are you using?

It was just a basic brain case. Oh and a few doors as well I suppose. Other than that, nothing.

Doesn't matter now though. I just went back and rebuilt the map from scratch. Problem solved. Thanks for the help though