The FoodSaver® Jar Sealer is designed for use with Ball® and Kerr® mason jars. Check the Jar Sealer to see if it is centered and pressed firmly over the mason jar lid. Make sure the rubber gasket on Jar Sealer is free from food materials. Examine rim of the mason jar for cracks or scratches. Any gaps can prevent jars from vacuuming properly. Check metal lid of mason jar to see if it is bent. Make sure Accessory Hose is securely in Accessory Port on appliance and on Jar Sealer. Try using two jar lids instead of one. Remove second lid from Jar Sealer when vacuum process is finished.