Lisa, I have the same problem, gunk coming up all the time.
We moved into this house after a family who had had quite a few animals, plus a lot of foot traffic. After cleaning, the carpet looks wonderful for all of two days at the most. Then the stains begin to come back, always in the same areas.
We assume that the dirt is down in the padding under the carpet. We cannot afford to remove the carpet right now, but you better believe that when we can this whole thing is going bye-bye!