does each machine have a dedicated charger or do they just plug in to first empty slot
if using any charger one bad plug can spread through the fleet very quickly. ie one charger plug partially melted will damage that battery which will damage that fork, next night that battery damages another charger whilst the original faulty charger starts on a new fork
do all the operators use the pull handle to seat the plugs correctly
is there a build up of dirt inside the forklift plug connector stopping the plug from going all the way in.
and as others have said correctly installing terminals on to cable is very important