Examine seal of bag. A wrinkle along the seal may cause leakage and allow air to enter. Simply cut bag and reseal. Sometimes moisture or food material (such as juices, grease, crumbs, powders, etc.) along seal prevents bag from sealing properly. Cut the bag open, wipe top inside of bag and reseal.
If you are vacuum sealing sharp food items, bag may have been punctured. Use a new bag if there is a hole. Cover sharp food items with a soft cushioning material and reseal.
See Foods that Should Not be Sealed for information on how fermentation or the release of natural gases from inside the foods may have occurred.