Maybe you are using the incorrect size crimp bead? You could look at the size string you are using and then look up what size crimp bead you should be using. Also, are you using a crimp tool or just bending the crimp bead?
Ditto what Stacy said above.Also are you maybe stringing to many wires through one crimp? Also do you use crimp beads (round) or crimp tubes? Tubes are better IMHO, especially if you crimp them properly.
Here is a tutorial on crimping properly with a crimping tool at about.com: