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.
Short strokes can happen to anyone at anytime, but it is not usually the fault of the weapon but of the shooter. Short cycling seems to occur more often when someone is under stress or in a cramped position. Trying to cycle the weapon while crouched or in the prone position can cause you not to draw back far enough to eject the empty shell. Come all the way back until you feel it stop, then reverse the action, count one then count two. One is back all the way and two is forward. Coming back ejects the shell; the forward movement chambers a round.
Engaging multiple targets will cause some shooters to short stroke because the brain is processing information to fast for the hands to keep up. This requires concentration and muscle memory. Your hands need to do the one, two stroke enough times where it is locked into to memory. You are on target and your mind says shoot but your hands have not caught up yet causing you to try to speed things up causing a short stroke.
Short stroking on must pump actions will result in a jammed weapon. This of course will cause problems in any situation. You can in many cases clear the weapon by trying to cycle it again. Some consider the modification made by Remington called the “Flexi Tab” a fix to a jammed weapon from a short stroke. The Flexi Tab is a modification of the carrier. The carrier has a small groove cut into it that allows more clearance for the shell, in other words, less likely to cause a jam from a short stroke. The modification allows the carrier to flex to prevent a jam otherwise, you would have to clear the jam by trying to manipulate the pump action or disassemble the weapon.
Your grip on the forend may be causing you problems as well. Gripping low on the action may cause the pump action to short stroke because you simply are not positioned properly to draw it back until it stops. You can only draw it back so far so some may naturally assume it is back far enough because their position only allows them to draw back so far. It has to come back until it stops, and you see the shell eject. In the heat of competition or battle you may not be paying attention to the shells but if, you do not see one you have to draw back farther and then reverse to make sure you chamber a round.