This video was created by Industrial3d for CNN showing a typical blowout preventer working properly and showing it not working properly. For example if the pipe rams were not closed the pipe might be bent or not centered and therefore could cause the shear rams to not work. If the pipe joint was in the area of the shear rams, the shear rams would not be able to cut through the pipe joint. Also, if the wall is too thick there would be problems too.