players can't hit mobs

I have a problem, from time to time players can’t hit mobs with sword or ax or fists.

they can only strike with the bow.

I am sorry for my bad english

What plugins do you have? There may be limitations.
If not with that how much ram do you have dedicated, or what are the average ping counts for the players? Those can all cause issues related to that.
Maybe an undeveloped plugin is not logging that the bow from the player hitting the mob, and just that a projectile hit the mob.
I could give you some recommendations if you want?
(Also, I like learning, who’s your host?)
Thanks for reading.