Midlew or mold on couch?

I bought this couch about a year and a half ago, we almost immediately (within about 2 months of purchasing it) noticed mildew starting to form under the cushions. I've done everything since then to try and remedy it, ive washed it with couch cleaner, vinegar, I take the cushions off often to let it air out, and we've moved apartments, thinking maybe it was just the humidity in our last place, to no avail. Our new apartment also has a AC unit with a dehumidifier setting that we leave on almost all the time, so im fairly certain its not due to humidity levels at this point. Anyone have any advice?