To avoid having unsealed jars again, it is important to know why they didn't seal properly the first time.
Here are some things that might have been to blame:
1) A chip in the rim of you jar:
Run your finger along the rim of each unsealed jar to see if you can feel any chips or nicks. If you find one, transfer the contents to another jar and reprocess it.
Any imperfections along the lip of the jar will prevent it from forming a good seal with the lid.
2) The jar wasn't filled properly:
Successful canning requires that there be a specific amount of headspace between the food and the top of the jar. This leaves room for the food to expand when it's heated and ensures that a proper vacuum seal is achieved.
Your canning recipe should specify how much space you need to leave at the top of each jar—it's typically somewhere between 1/4 inch and 1 inch. If you don't have enough food to fill up the last jar, just stick it in the fridge, and use it up first.
3) The top of the jar wasn't clean:
After you fill your jars, it's important to wipe the lip clean, so it'll form a tight seal with the lid. If you missed this step, just clean the lip before you reprocess the jar.
4) The lid wasn't centered:
Canning jar lids have a sealing compound around the bottom edge that's designed to make contact with the jar's rim. If this is too far out of alignment, the seal might not hold. (This compound is also the reason canning lids can't be reused.)
5) The ring was too tight:
When tightening the ring onto the jar, it needs to be secure, but not too tight. Air needs to be able to escape during the canning process, and a super tight ring may not allow this to occur.
Cutting the canning time short, or failing to keep your canner at the proper temperature/pressure the entire time you were processing your jars may have prevented a proper vacuum seal from forming.
You didn't make the necessary time/temperature and pressure adjustments for your altitude: If you're canning at a high altitude, you'll need to make adjustments to the canning instructions provided in the recipe. Occasionally a recipe includes this information, but more often than not, you have to calculate the changes yourself.
It's possible that you accidentally skipped an important step.
1) Check each jar for nicks along the rim.
2) If you find a nick, transfer the contents to a new jar. If you don't find any nicks, put a fresh lid on the jar and secure it with a ring.
3) Reprocess the jars using the same processing time as before.
4) Allow the jars to cool.
5) Then, check for a good seal.
After removing jars from the canner, you should hear a 'ping' as each jar cools.
This sound is a good indication that the jar has sealed properly.