Sounds like you are trying to crosscut an angle that is greater than the swing of your miter saw. Some saws will swing up to and some beyond 50 degrees, others stop at 45. To cut an angle greater than the swing of your saw, you have to make a temporary fence that is at an angle. You set the saw to cut 90 degrees, which is straight out, then attach a fence at the 90 degree compliment of the angle you want to cut. You have to provide support for the board since it will be held somewhat perpendicular to the normal saw fence. Clamp the board/molding so it doesn't move and keeps your fingers clear.