One of the biggest differences between forehand and backhand looping is that, for the forehand, the body isn't in the way. This means it's relatively easy to take the ball early, middle, or late in your hitting zone. This makes timing a lot easier.
On the backhand, the body is more in the way, and so you can't adjust the contact point as easily. This means that if you are rushed, you can't that easily just take the ball late. Result? Many players get jammed on the backhand, perhaps by a quick, deep push, and so rush their backswings. Result? Lots of erratic misses.
Early racket preparation is key to the backhand loop. If you hold your racket too high, you won't have time to bring it down into position. If you stand too jammed to the table you also won't have time. Instead, find the perfect ready position for yourself, where you hold the racket low enough so you don't have to rush, and not so close to the table that you get jammed. (Arm's length is usually about right.) You should be able to bring the racket down and back in a smooth motion, and then whip it through the ball, giving the ball an arcing topspin - just like the pros. At the back of the backswing, the racket should actually come to a brief stop as the rest of the arm begins its forward motion (with the arm and elbow pulling), leading to that whiplike motion as you put the racket through the ball, almost like throwing a frisbee in a slightly upward direction. If you don't feel that slight hesitation as your racket reaches the back of its swing, then you are rushing your backswing.
Some of you don't have a backhand loop. Then, may I suggest developing one?
Note that everything here should also be hints on the reverse - how to play a player with a good backhand loop. In particular, this means rushing him with quick, deep pushes, and bringing him in with short serves and pushes and then jamming him with deep pushes.