battlefield 4 on xbox one issuses

when playing battlefield 4 i run into some players that when i unload a whole mag into them they still have 100% health even the hit markers show that i am hitting them and they kill me then when i re-spawn i can kill then i don't know what is causing this but i am getting disappointed because my skill level has dropped from 200 to 106 even when i do good in a match and go positive in TDM it still drops it's getting to the point i don't even want to play battlefield 4 anymore and it's one of my favorite games so if i can any answers and help over it i would greatly appreciate it