There could be a number of reasons. The Fed chop shot is probably best when used against a ball that is hit with enough pace. Fed isn't generating much of his own pace, just slowing the ball and slicing it crosscourt and low.
You probably don't have Fed's eye-hand coordination, so your racket face is not at the right angle and you're not getting the racket speed through contact.
Lastly, you probably just over-estimate your own playing ability and need for practice. Change the racket face angle if you're hitting them into the net.