Short stroking or short cycling means you have not brought the bolt back far enough to eject the cartridge, which means you cannot chamber a shell. In some instances, the shell may eject but there is not enough force to chamber a round and it can jam on you, which is not what you want to happen. On semi-automatics, the work is done by the exploding gas from the propellant or gunpowder. Shells designed for less recoil can cause a short stroke on semi-automatics because there is not enough energy produced by the powder to slam the bolt back and then propel it forward. The work is done by you with a pump action shotgun however. Sometimes not allowing the trigger to reset or there is a failure to reset can cause a short stroke but this is not typical.