i've encountered this issue a number of times: after fully charging up the hammer and swinging at an enemy, the swinging animation won't play out properly and end up having no effect.
and given that sledge has to be in melee range to make use of his gadget, missing your swing can be detrimental.
on that note, i feel like sledge is one of the worst operators. not particularly bad, but definitely on the lower end and could use some tweaks. maybe make him able to sprint while holding the hammer, as he's painfully slow while holding it and is outrunned by everything apart from a smasher, which brings a very high risk given that sledge has to fully charge the swing every time to stun an enemy, where it would just be better to shoot them or use anything else.
it'd be also better IMO if he could equip it as a separate weapon entirely because on repeated swings, if you mistime your next swing, he will holster the hammer and draw his gun and you'll have to go through the hammer equipping animation again wasting precious time.