Welcome to the forum.
Here are a few links to this problem that have been discussed in the past. Check them out and if they don't help or you have any more questions don't hesitate to ask.
Al

http://www.poolforum.com/pf2/showthread.php?t=1680

http://www.poolforum.com/pf2/showthread.php?t=764

http://www.poolforum.com/pf2/showthread.php?t=141