Except at the advanced levels, few players really attack the middle effectively. What is the middle? It is roughly the opponent's playing elbow, midway between forehand and backhand, where the opponent has to make a quick decision on which to use; move into position to do so; and then try to recover for the next shot, since they are now out of position, leaving one of the corners open. Almost all of your deep shots should go to the wide corners or the middle. Corners are easier to play because they are not a moving target, but the middle is often the weakest spot for an opponent. Many think they play the middle, but what they think is playing the middle often isn't.
For example, their crosscourt backhand "to the middle" might cross the table in the middle, but by the time it reaches the opponent, it's moved diagonally into their backhand court. (So they need to aim the shot so it reaches the middle as it reaches the opponent.) Or a shot that seems to go to the middle just feeds a moving opponent's forehand attack off that middle shot. (So they need to judge better when to go to the middle, and perhaps do so more aggressively, or more towards the opponent's backhand side, since he's moving to favor his forehand.) Or a player might start a rally by attacking to a corner, planning on going to the middle on the next shot – but faced with a strong return, are unable to make a strong shot to the middle. (So they need to play the first attack at the middle, which often sets up a follow-up attack or put-away to a corner.)
So . . . are you really playing the middle? Here's a simple test - if the opponent is a bit awkward on covering your shot to his middle, then you've found the middle. If he doesn't, you haven't.