This problem can be caused by a few different issues:
- The jar lid was not completely sealed. Inspect the lid for a bad glue ring and inspect the glass surface for imperfections.
- The fill temperature was too low. Did you fill at a minimum of 190 degrees F°?
- The container wasn’t sterile. Did you clean your jars and lids thoroughly?
- The sugar content was too low. Sugar is a natural preservative, therefore, if you want to use less than 40% sugar I recommend our Pacific LM-O Pectin, which contains preservatives that will lengthen shelf life after opening.