death web and facets both have -res.
the person you are attacking has x res on screen.
if the -res on deathweb etc bring them below that amount then they will take extra damage.
However, if the opponent has extra res above and beyond the displayed res (stacked res) then even when you take res off their total res it wont change their actual in game res so no extra damage will be taken.
The way to deal more damage to them is to increase your damage rather than decrease their resistance.