Originally posted by tomoscar:

Could it be that the skin of creatures function as armor, and that damage delivered has to be higher than the armor's protection value, for the creature to take any damage?
(Damage taken = damage delivered minus armor value)
If so, the Valheim wiki should contain a value for the creatures skin armor.

no. you're definitely encountering something weird. move closer to the lox to check its hp bar and see what's going on.