To get the proper crease, you need to have the correct female channel width, channel height, rule width and rule height. In most commercial printing and bindery applications, the rule width and height won't change (usually a 2pt) unless you're running very light or very heavy stocks. The female channel width and height however will change with almost every job, depending on the caliper and density of stock.
If you're new to this, you'll want to familiarize yourself with a couple of creasing matrix solutions: Shreiner Creasing Matrix [www.shreinerco.com] and Cito System [www.cito.de/en/GB/] With the right material, you should be able to quickly eliminate almost any cracking. Good luck!
Andre Palko
http://www.BinderySuccess.com