Either in the build, save, or at least "check for errors".
The filename must be between 5-8 chars.
If it is 9 chars, it will not report a problem saving, or during error check, or during build.
Trying to open the terrain in offline mode for testing just barfs and loads CraterMA instead with no error msg giving you a clue why.
Strangely, a Staged mission will still work with the terrain just fine.
I had hit this once before so it finally dawned on me what the problem probably was. I can't image how many novice map builders made this mistake and threw their hands up in frustration.