There are so many pieces that go into being a great pool player that when things are going wrong, sometimes it's difficult to pinpoint where the problem is. When I am struggling at the pool table I will usually revert to a few simple drills to see if my problem is a mechanical one. Regardless of how well you know the game or your skill level, how well you perform really comes down to one simple thing...delivering the cueball to the intended spot. If you're struggling with your delivery, here are a few drills that will get your arm back in the slot, and get your cueball back on its leash.
In this drill the cueball and object ball are both at the middle diamond and about 1/2 inch off the long rail. The idea here is to pocket the object ball, and let the cueball drift up just enough to replace the spot where the object ball was. This drill will not only help you align and hit center ball, it will soften your stroke. Hitting this shot too hard will create a stop shot, and hitting it too soft will allow the cueball to follow. The perfect delivery will only allow the cueball to roll up a couple of inches.
This drill is the same one as above, except that instead of letting your cueball drift forward you have to draw the cueball to the back rail. This shot requires excellent aim, and a very pure stroke. Some things to remember on this one are to hit the cueball low, keep the cue very level, and keep your wrist and hand very loose. A long, smooth follow through is essential.
This torturous little drill was one I started doing when I was struggling with rail shots. This drill does not focus on position at all, but it will definitely hone your aim and get you hitting dead center on the cueball. Throw the object balls randomly on the table and place the cueball frozen on the middle diamond of the short rail. After every shot, return the cueball to this position. If any object balls are on the opposing short rail, move your cueball left or right to create a possible shot, but keep it frozen to the back rail.
These three drills should get your cueball delivery in good working order. Once you are hitting the cueball where you intend to, you will know if your problem is a visual one such as picking the wrong contact point, or a mental one like changing aim over the ball. Whether your game needs fixing or not, it is always good to do drills like these regularly to keep your alignment in check, and stop little inconsistencies from working their way into your stroke.