Our dog ate a large amount of spreadable butter (butter blended with canola oil). He threw it up on the carpet. I cleaned with our spot bot, weeks later the spot began to reappear as a dark spot.
Sounds like you have a bad stain on your hands! When spots reappear, that’s a sign that the spot or stain has gone into the padding below. Here’s what you should do; pretreat the stain with an enzyme action based product such as Pet Stain & Odor Pre-treat for Carpet & Upholstery
for 48 hours. Then give your stubborn stain one more go with your upright carpet cleaner
To make sure you reach the padding of your carpet, run your carpet cleaner back and forth over the spot until you no longer see color coming up from the clear nozzle on your carpet cleaner.
After carpet cleaning, if spots happen to come back again, just touch it up with OxyPro®