So I have been using the Shadow Hunter and Sharpshooter mainly. I used the Sharpshooter to snipe headshots exclusively as it always was an insta kill. But I messed around with the Hunter bow for awhile and found it would insta-kill headshot as well.

Before I had green and blue mods but I have been getting more purple damage mods lately and my damage suddenly fell off a cliff. Don't know if its a glitch but before I was doing 70-110 damage before the mods, and now it maxes out at 50 damage.

Maybe the enemies just get more HP later in the game but now the Hunter bow is pretty much useless in any fight for me other than to overheat an enemy.

What kind of enemies are you fighting? There are a lot of enemies that are resistant to the default arrow type and you'll need to switch to Impact arrows to do any real damage if you are using the Hunter Bow. I find the Hunter bow is still a great bow but as the enemies get tougher the tactics for bringing enemies down needs to change drastically.

I used to be able to one-shot Watchers if I headshot them with the Hunter bow but now it only takes off 50% health. Thats when I first noticed the damage difference. I have switched to Impact arrows now instead of the base ones and it makes a big difference. I thought they were only for knocking off armor which is why I never used them before. The fire rate is the only reason I keep using the Hunter bow as the Sharpshooter loses accuracy after the first shot so bad that every shot after goes wild.

The late game human enemies I can understand being tougher headshot cause they are wearing helmets now. I thought I saw an article online about this glitch but maybe it was patched since then.

I don't get head shots on anyone but the weak, unarmored enemies and the regular arrows, no matter the bow, don't do the trick late game with many enemies.

Not sure about the aiming end of this but a note on the watchers. There are a handful of different Watcher "variants" from what I can tell, and certain ones do take more hits to kill. Im not sure if it is zone based, but later on many watchers will take more than what your hunter bow can dish out (unless you are stringing 2 arrows at once). Hope that is useful to the damage end.

Patch just came out this morning in the US so maybe check that but yeah might be a glitch unless they are corrupted watches. Don't think you can one shot them in the eyes with arrows.

"Redeye" watchers take more hits, even with direct hits to the 'eye.'

Oh yeah I forgot about those ones. They can still can be one-shot if you have double or triple arrow ability unlocked but that's a playstyle choice.