Players so often hear coaches tell them to attack the opponent's middle. (The middle is not the middle of the table; it's the midpoint between your forehand and backhand, where your playing elbow is.) But it is equally important to be able to cover the middle when the opponent goes there. How do you do that?
First, remember the "Middle Rule." In general, if the ball comes to your middle and you are rushed or close to the table, favor your backhand; if you have time or are not too close to the table, favor your forehand. You can vary this rule based on your own individual skills and playing style, but in general it's a pretty good rule to go by.
However, there is more you can do against those middle balls. Don't think of them as a problem; think of them as an opportunity. If you react properly and in time, you have several advantages.
If you are using your backhand close to the table, you can do a quick attack to both wide angles, as well as the opponent's middle. Often it's the perfect time to go quick to the opponent's forehand, drawing them out of position. If they see that coming, they may move to cover their forehand - in which case they may be open on the wide backhand.
If you are using your forehand, here's your chance to dominate the table. First, just like with the backhand, you now have an angle into both wide corners, as well as into the middle. Second, the shot will leave you in perfect forehand position for the next shot, so if you are strong on the forehand, you probably will get two forehands in a row.
Whether you use your forehand or backhand, you always have the option of going right back at the opponent's middle, and if your shot there is stronger than their shot to your middle, you should have the initiative.
So learn to cover your middle and turn this normal weakness into a strength. Practice this with random drills where your partner puts the ball to all parts of the table - both wide angles and your middle - and you practice making strong returns against them. Another good drill is to have your practice partner alternate hitting one randomly to one of the wide angles, then to your middle. You respond to the random wide angle shots with the appropriate shot, and then move to cover the middle shot with your forehand so that you learn how to dominate the table with the forehand from the middle. (You can also do both of these drills, and many others, multiball style.) Or make up your own drills to learn to cover the middle - there are many possibilities. Go to it.