Attribute applies to melee pet dmg even if it is not the element from their skills. For example, with 150 fire AM on a pony, you'd still get 75% (or 80% (?) depends on chronicle) for melee dmg but 0 attribute added to hydro screw.
Check with a different mob, probably it is a resist issue but it should add to melee dmg even if it is not fire elem in your case.