You may not always hear the ping. The way to make sure that the jars have sealed is to check the lids after a few hours. Press your finger in the center of the lid. If the lid makes a popping sound and it pops back up after you remove you finger, put the jar into the refrigerator and eat the contents within a few days.

Don't reuse the lids anymore- lids that have sealed once may not hold a seal again, or worse yet, may come unsealed after some time. The lids are one-use, unless you're using Tattler lids. Tattlers can be re-used (although the rubber gaskets eventually need to be replaced) and they don't make a popping noise when they seal.