the issue I had was that the engine needed a wait (1) directly after the terrain creation command to do some sort of an update ..

it may very well be that your problem is a level size issue like sivan said , because you mentioned a single terrain being "loaded" , mine were created, but I added the issue here because it sounded so much alike , I had the same problem although perhaps under a different situation .

The c_setminmax was also a terrain creation /modification issue I had , just thought I would mention it too .

good luck.

edit : just saw your new reply , "multiple terrains" put a wait (1) after each