Something is wrong with PvP. Is it lag?

Hello! I have encountered an error on my server. Whenever anyone is pvping, sometimes they randomly wont be able to hit eachother. This lasts for a few seconds. It doesn't seem like lag because they are still able to talk and do other stuff. What is going on here? Could it be a plugin? Please help!

 

lag. Whats your ram and do you have a good internet connection for the server and the players fighting?

 

It's very good actually. I'm running on a 16GB dedicated server with 12GB allocated

 

alright do all of the players have a good internet connection?

 

Yep. No one has less than 600 ping unless it's their own internet. Those who have higher ping are in australia

 

Do you have pvp=true in the server.properties?
If you do, It may be a plugin blocking it. If this is the case, please post a list of your plugins.

 

Of course I do. I said it was lagging/glitching if you would have ready my whole post. I'll post all of my plugins in a bit