Sometimes when i just stand completely still, and i scope in on the enemies head and tap once, the bullet hit a pixel behind him and doesnt register, why does this happen, the enemy is standing still too

I'm stuck in low immortal and always bottom frag, I aim train but everyone still has better aim than me. I try to vod review but am unsure what I'm doing wrong I try to play sentinel or initiators to help out the team. While playing those...