Hello there...
If you have several maps installed besides the default ones, check one by one (.pk3) at the /texture/ folder... sometimes, some maps includes additional textures for other maps... for example:
Suppose there is a map named "xyz.pk3" but inside of its /textures/ folder, you will find /textures/xyz and /texture/tremor.
At netradiant (or gtkradiant) you will only see "tremor", so be sure that any texture you use is actually inside the pk3 you are using or inside the texture folder.
I hope that help...