Make sure the CREATE-MESHES tick-box in WED is turned ON when compiling a level.
(this tick-box is in the advanced settings page of the compiler panel.

If that still doesnt help, try deleting ALL the level 'object' files except the WMP file itself,
then recompile in case there are some junk settings or corruption in one of those
other files.

By 'object' files, I mean xxx.WMB, xxx.$$M, xxx.$$W, xxx.BAK, xxx.RAW, and xxx.WED.
where XXX is the name of the level.