A. Failure of lids to seal may be caused by one or more of the following:
•A chip on the rim of the jar.
•Failure to follow manufacturer's directions for preparing flats.
•Presence of food particles/liquid on jar rim. Always wipe rim clean before putting on lids.
•Leaving too little headspace, letting pressure in pressure canner fluctuate, or lowering the pressure too suddenly. In all of these cases, food particles/liquid may be forced between the jar and lid, causing sealing failure.
•Leaving too much headspace may prevent sealing because the processing time was not long enough to exhaust all the air from the inside of the jar.
•Not removing air bubbles can have the same effect as leaving too much headspace.
•Putting screw bands on too tightly.
•Tightening screw bands after removing jars from canner.
•Reusing lids. Use flat metal lids only once.
•Using commercial or one-trip jars, such as for mayonnaise, peanut butter, or baby food. These jars have slightly different sizes of sealing edges and are not recommended for home canning/preserving in general. These jars are acceptable to reuse providing you use new lids appropriate for the particular jar. Some of these types of jars are only suitable really for preserving in a boiling water bath type pot or FV unit as the jars themselves are thinner than the jars used in the pressure canner. It is recommended that anyone reusing jars takes into consideration the type of jar, what it was used for and the thickness of the jar before reusing it for pressure canning.
Be aware and be safe