In my last article I wrote about the mirror image banking system, which gives you a great reference point for banking balls, but whether you use that system or another one, certain forces of physics will change your perfect angle to the pocket. There are many variables that affect the angle of a bank; collision induced throw, cushion induced spin, rail compression...I won't go into the science of colliding spheres, but I will give you the info that you need to identify the trouble banks and make the adjustment to get them back on line.
1. Object balls that are close to the rail. When an object ball is close to the rail the bank will tend to come short. The reason for this is extra rail compression. To counteract this effect either use outside spin (if the cueball is cutting across the ball from right to left use left spin, and if it's left to right use right spin). If spin will make you lose position, then cut the bank more.
2. Banks that require a big cut across the ball. Banks that are not "natural", such as when the cueball  has a large cut across the object ball will change the angle because of collision induced throw. Collision induced throw is what happens when the balls connect and the friction between them actually pulls the object ball in the same direction as the cue ball. The dirtier the balls, the more throw you can expect. The correction is outside spin, or cut the ball more.
3. Banks that look too wide. Some banks look like they will go too wide, but if you try to hit them where they need to be hit you will double kiss. For banks like this you may be able to shorten them dramatically by hitting a stop shot using with a very hard hit or inside spin (if you are banking a ball to the right, use right spin, banking to the left, use left spin). If you need to shorten it a lot, use a combination of both spin and speed.
4. The type of table you are playing on. The diamond systems run very true on a Brunswick or Olhausen, but if you are playing on a Diamond you may find that the rails are consistently running short. In that case use outside spin and/or a softer hit. If you're playing on a very old table with bad rails you may need to add extra speed and/or inside spin to make your bank shorten up. 
5. New cloth. No matter which type of table you play on, when it first gets recovered you may think you're just having a really off day. What's actually going on here is slide, and lots of it. The same friction that causes throw is almost non existent, so the balls will just slide around making banks and kicks go very wide. On new cloth, if you need to widen a bank, outside spin will work just fine, however, if you need to shorten a bank it's better to just add speed. New cloth that's slidey refuses to accept any reverse spin (spin that goes against the natural angle).
Banking a ball in when your opponent thinks you're dead is powerful thing, so it's worth the effort to learn it. Some people just have a natural feel for banks, but if you're not one of those people (I wasn't!), don't despair. Take these principles to the practice table and you'll be banking the game ball down your opponents throat in no time.