controll not stoping when i kept break-point in user exit.

dear,

In MB01 transaction we impemented the user exit .now i want to do some changes in it. so, i want stop the program control when it comes to user exit .i kept the break point in user exit and i tried by writting the break-point also. even though control not stoping in user exit . plz tell me any other way to stop the control in user exit.