This is an issue that's kinda hard to deal with, as everyone sees this differently.
first you have to take into account lag- as with any amount of lag/ping (i never remember which is the right one on this topic) you will see things 'slightly' differently to the server, and with a precision weapon like a bow or spear that slight difference might mean the difference between a hit or a miss.
also take into account your point of view- it might LOOK like you hit them when you are looking down from atop a horse, or in a chaotic fight, but in fact it was inches from their face, or just past their hip.
MANY times I've had a lancer/spearman call bull**** on me, when from my point of view the spear tip was nowhere near me.
On the other hand, I've experienced the other side of this too- when i could swear hands down my spear hit them but no damage was dealt. it happens fairly infrequently for me (i use spears a lot btw) and when it does happen it tends to be in high ping games, or when im not really paying attention.