Could be a number of things.
Reverse isn't synchronized, but all forward gears are. If you have a hydraulic problem, and the clutch isn't completely disengaging, often it'll be tough to get into reverse. Does it grind at all when you try to get into reverse?
Have you tried double clutching it before putting it into reverse? Basically shift into neutral with the car stopped, let out the clutch, then push the clutch in and shift into reverse. Does it get better? Putting it in neutral with the clutch out (and the car stopped) will spin up the input shaft and can sometimes help align the dog clutch to the splines.
It might just be shifter bushings (at the shifter or on the transmission). If things get squishy and the cable aren't correctly lined up, it can get hard to shift.
RF