It’s the anti cheat. Basically you’re doing way too much damage in a set time, so the anti cheat kicks in, and they take no damage.
It could also be the lag, basically if someone has 200 ping, they’re 200 ms from where they are, so it doesn’t count since you’re not hitting him, just the lagged version of him.
To prevent this you could space out your melee attacks more or go in front of the player and hit him from there