Hello, this issue happens when the other player you are playing with has disconnected from the relay server. You should notice that you are blue with them, and they are blue with you, but when the player is not on the relay server, and is blue to someone, he or she can only receive the UDP packets from the player that is connected to the server, thus allowing them to hit you, causing no damage to them because they cannot receive your hits.
~RedEyes.