There are a few main ways that enemies can block or reduce damage:
- Shields: each layer of shield nullifies one instance of damage
- Iron Skin (X): reduces normal damage by X. Doesn't affect damage from other enemies, poison, freezing, ...
- Frozen: a frozen enemy doesn't take any damage, but hitting them will remove the Frozen effect.
- Health Gates: these are choke-points where you can't get past them with 1 action (like a card). Only used on the bosses at the moment
Maybe one of these was reducing the damage? I suspect Iron Skin, it's quite easy to miss...