Net Radiant/GTK Radiant? What OS? You sure the brush is in the area shown on the grid? In preferences->Orthographic check 'Display size info'. In Misc menu->Colors what color are 'Default Brush' and 'Selected Brush'? You can press 9 to only show bigger lines on grid and then zoom out (mouse wheel, use 1-9 to change grid size, and 0 to show/hide it). Are there any errors in console (press 'o')? Something here ought to help...
Btw: in preferences->Brush check 'always use caulk for new brushes', and if Net Radiant then in pref->Clipper check 'Clipper tool uses caulk'.