Maybe a receptor resistance build up? Seems to be a common pattern that medication needs to be dosed up to work- and blunts the receptors for the "normal" amount the body produces, if it does not cease to produce it at all.
Don't sNRIs increase the availability of norepinephrine rather than decrease it? Which is effectively the equivalent of producing more, not less?
But… I’m not sure it really works that simply in a real brain.