In addition to the above questions,
When you put your draft barriers up, how much ventilation space did you leave and where is that ventilation located?
What's the groundwater and drainage situation? What's the weather been like?
How deep is the mulch? Is it loose and fluffy or has it packed and matted?
With the description of "rotten cabbage" my first instinct is to think that the bedding has gotten both wet and packed so that anaerobic pockets have formed.
Preventing packing/matting is why I favor a mix of bedding materials with different compositions and textures -- wood chips, wood shavings, straw, pine straw, leaves, etc.
Throwing a handful or two of scratch into the bedding a couple times a week enlists the power of chicken labor to mix and turn the bedding, but it still may be necessary to get in there with a pitchfork to stir and fluff the trouble spots (under perches, at the end of the ramp, near the feeder, etc).