It's possibly not the hit detection. It may be a ping and/or buggy killcam problem.
After you die, in a situation like you mentioned above, quickly check your ping and the ping of your assassin. If they're way off from eachother then that could have been the problem. Also, killcam is very bugged still and not all that accurate. I would only rely on killcam for certain easy kills. Don't let it get you down, I have those same type of matches... do GREAT one match, then bomb the next. It just means we aren't pro lol