It sounds like one of two possible problems.
1) you are using a bend radius that too tight for the alloy you are using. Certain heat treatable and non heat treatable alloys require a larger bend radius to prevent cracking. For instance, while carbon steel may exhibit 25% elongation, some aluminum alloys will have limited ductility and crack when 15 to 20 percent elongation is exceeded.
2) you are attempting to bend parallel to the direction of rolling. Try a couple of practice bends to see which direction is most favorable. The bends should be orientating 90 degrees from each other to determine what the direction of rolling is.
Good luck. Al