Air was removed from the bag but has now re-entered.

Examine the bag seal. A wrinkle along the seal may cause leakage and allow air to enter. Simply cut the bag and perform another vacuum. Sometimes moisture or food(such as juices, grease, crumbs, dust, etc.) along the seal will prevent the bag from closing properly. Cut the bag to open it, smooth out the upper part of the bag and carry out another cycle. If you continue to experience difficulties with air in the bag, you may have experienced fermentation or the release of natural gases inside the food. When this happens, it is possible that the food has begun to deteriorate and must be discarded.