Can't deal damage to a certain player

I have a factions PVP server running on bukkit with plugins: factions, essentials, PEX, and Infected. My friend and I can take and deal damage to other players but we can not hurt each other. When we try to hurt each other nothing happens at all. We are not in a faction, no godmode, etc.

Is there any way to fix this? I am clueless on what to do.

 

Sounds like its a setting of one of your plugins - perhaps a permission node or a config option.

Try removing your plugins and testing. Then add them back in a couple at a time until you reproduce your problem - this will allow you to narrow down to which plugin is causing this issue for you. Once you know that, you can start looking into the configuration file, or the permissions to see if you can spot the problem.