what type of food is the eel being fed and how often.it could be a result of a few different things such as not enough nutrients given.food to large could dislodge the jaw itself.damage due to rocks or impact.
I see alot of refernces to eels in campairison to snakes.they are in no way compariable other than being long.snakes do have jaw sockets that can be dislocated to swallow the prey .eels do not have this ability.but what they do have in common is what every reptile,mammel and fish has in common.lack of nutrients can cause lockjaw or deformation of growing joints this is a true sympton of malnutrition in alot of different creatures including humans.if you would like to think of them as a reptile anyone hat has owned reptiles should know the effects of lets say calcium in eguannas this causes deformed jaws and joints or lack or other needed nutrients.if hes still eating well try adding some selcon to its food before feeding this will give a little more nutritional values to all foods. and smaller portion sized foods