Recipe: Why jars don't seal

Many times it is because they are overtightened. Just tighten the bands until you feel a slight resistance, only finger tight. Also, they should only be placed in 180 degree water, not allowed to boil.

Here is some more info:

There are several reasons why lids do not form a seal on jars after you process them. The jar and lid have one purpose: to make a seal at the time of processing. This seal does not guarantee that the food will not spoil; it only keeps out contamination. Each manufacturer uses a different composition for the sealing compound around the edge of the lid. One common reason why lids fail to seal is because you may not have followed the directions on the package. Other failures may be due to:

If a lid fails to seal on a jar, remove the lid and check the jar-sealing surface for tiny nicks. If necessary, change the jar, add a new, properly prepared lid, and reprocess within 24 hours using the same processing time. Headspace in unsealed jars may be adjusted to 1 1/2 inches and jars could be frozen instead of reprocessed. Food in single unsealed jars could be stored in the refrigerator and consumed within several days.

Under or overtightening of the screw band.
Jars with nicks, cracks or chips on the sealing edge.
Reuse of single-use lids or jars.
Incorrect amount of headspace or failure to remove air bubbles before placing lid on jar.
Faulty or insufficient processing that does not destroy the organisms that cause spoilage. The lids may seal at first, but then unseal later as spoilage occurs.
Failure to clean the rim of the filled jar before sealing. Uneven processing temperatures can allow food, grease, or seeds to lodge under lids when liquid leaks from the jar during processing.
Careless handling of jars and lids before and after processing.

Tightening or loosening bands immediately after processing. Never take for granted that all home canning lids and caps are alike and use exactly the same way. This is a common cause of sealing failure.