The middle is the weakest spot for most players. When I say middle, I don't mean the middle of the table; it's the mid-point between the opponent's forehand and backhand, usually where the playing elbow is. Most beginning players play to the opponent's forehand or backhand, while most intermediate players play wider angles. Advanced players know that while angled attacks are good, the best place for the first attack (and often follow-up attacks) is right at the middle.
Why is the middle so weak when attacked? There are five (yes, five!) primary reasons.
Attacking the middle isn't just an option; it should be the default place to attack unless you have a reason to go elsewhere, such as an open corner, a slow opponent who doesn't cover the corners well, or an opponent who is weaker on one side. The nice thing about attacking the middle is that even if the opponent knows it's coming he can't really prepare for it. If he tries to favor one side to cover it, you simply move your target over to compensate.
While you should usually be aggressive when going to the middle, there are at least two other times you might go to the middle with a less aggressive shot. First, against a two-winged attacker, you might serve or push deep to the middle to make him decide whether to forehand or backhand attack and then move to do so. Second, going to the middle cuts off the angles, so a passive shot can't be attacked at a wide angle. (But a smart opponent might instead attack your middle!)
Why don't players attack the middle more often? There are two primary reasons. First, it's difficult to hit because it's a moving target. When you attack the wide forehand or wide backhand, it's the same place no matter who you play. But the middle changes not only from player to player but throughout the rally against the same opponent, depending on where he is standing or looking to do. (An example of the latter is if the opponent is looking to attack with his forehand, then going to his middle only gives him an easier forehand, so his "middle" moves toward his backhand side.) The only way to develop the ability to attack this moving target is by doing it over and over in matches and drills until it becomes second nature. And this leads to the second reason players don't attack the middle more often: because they don't practice attacking the middle.
Most players practice their attacks either crosscourt or down the line to their partner's block, and so they go to the partner's forehand or backhand, over and over. This becomes a habit. If all one does is attack to the forehand or backhand in practice, how likely is the player going to go to the middle in a match? You have to practice it if you want to do it in a match. Here are some drills you can do to practice attacking the middle.
So learn to attack the middle, and soon you'll leave the middle of the pack as you move up the ladder of table tennis success!