are you using opengl to play srb2?
wadauthors internal nodebuilder isnt exactly the best ever, it tends to make walls invisible in opengl mode, and are you using the thok barrier?
if you are then you do not put the texture in the "main" slot, you put it in the "below" slot, and make sure that the linedef side you are editing is correct, for an upwards standing platform, edit the back side of each linedef, for a pit, or anything within the thok barrier, edit the front side.
anyway, glad you could make it here! :)