Why do i always get ripped off in halo reach for no reason?

i shoot the guy in the head 3 or 4times and hit him with my gun and he dont die and halo 3 that would be instantaneous death, and i even tryed other tactics like shooting them in the head and throwing grenades under their feet and then hitting them and does it do somthing no not really only somtimes. and i dnt know why i bother playing the arena when its to easy i should just stick with mlg playlist to get better like the big dogs. i went negative 1 one match just because of bologna like that i always . or ill throw 2 grenades at somone and they will be standing ontop of yet they dont die. and its not lagging because everyone is moving perfectly fine.