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 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 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 screwbands on too tightly. Tightening screwbands after removing jars from canner. Reusing lids. Use flat metal lids only once. Defective lids. 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 from home canning.