If you are trying to shift from 5th, straight into reverse (while stopped, of course), then you can't. There is a reverse lockout that prevents that. All you have to do is shift from 5th, into neutral, then let the shifter go into the center position (this "unlocks" the lockout), then you can move the shifter back over and into reverse.
I'm assuming this isn't the problem, but I'm just throwing it out there, just in case.