Just an addition to Vemonus' answer.
I'm playing as well, and I noticed that when
Aim Assist was on in the
Settings menu, sometimes the arrows would change their course mid-flight and hit something else. You may be aiming for the
Blaze canister, but right before it hits it may change course and hit the body instead.
Another thing that I noticed is that the higher I leveled up, the more armored the enemies became. When I first encountered a
Snapmaw I was able to hit the
Blaze canisters near its head pretty easily. But after I got up to around level 20 or 30, I noticed that the
Blaze canisters were now armored, requiring me to shoot off the armor before being able to trigger an explosion with a fire arrow.
I don't know if this coincided with my level increasing, or getting a Carja/Shadow Sharpshot/Hunter Bow, which gave me new ammo types that are better at shooting off pieces of armor, or even if the difficulty of the machines increases as you go farther away from the Sacred Lands.