When sitting still with the engine running and the clutch out, try pushing the clutch in, wait 2 seconds, then try to engage reverse. If it doesn't go in smoothly, you might have something keeping the input shaft spinning with the clutch disengaged (or the clutch isn't completely disengaged)...
The only other things I can think of is that the synchros are going in the trans or the shift rods/rails/bushings have become sloppy...
Do the gears clash or does the shifter just stick and not go into gear?