I assume the characters are not very high in level. Is that the case? I have noticed the same thing in the games I am playing in; the low level characters have such low bonuses that even when the target is stunned there is no guarantee of a hit, non-the-less a killing blow. Once the characters of a decent level, with good bonuses, this should mostly correct itself.
If the target is completely overwhelmed, the attackers should also be able to outmaneuver and at least flank, maybe get someone behind the foe, get around a shield, etc, so don't forget to add those things too.
I generally have not had to stop a combat before it's done (surrender counts as done), but I have handwaved past all the initiative and non-attack actions just to quickly roll a couple rounds of attacks to settle things. In principle, you can call it over, but it just doesn't feel as satisfying.
I do tend to have foes not parry. No parrying increases the intensity of the fight and one way or another it will be over quickly. Foes parry when there is a specific tactical reason (e.g. an important individual is temporarily advantaged and needs to buy a round or two), but outside of that it lets the PCs decide the intensity of the engagement.
On the other side of things, give the foes a healer and those enemies who are not actually dead may also not really be out of the fight. So that addresses things in an opposite way....