Our inventory system works on a combinations of identifiers including UPC and system generated values for each individual product. This means that when these combination of values mismatch between Gateway and the customer facing site inventory cannot be updated. For this reason it is very important to not attempt to change the UPC or universal identification number for a product that has been live on the Groupon.com.
If you have already updated UPCs or changed UPCs then you may have already noticed that inventory changes made in Gateway are not being reflected on Groupon.com. To resolve this you will need to remove the affected product and then re-create the product.