Speaking from experience, there is nothing more frustrating than finishing a batch of canned goods only to find that some, or even all, of your jars didn’t seal.
While there are a variety of reasons your jars might not be sealing, I’m going to focus on issues with the main components: the jars, the lids and the rings.
The Jars
While this might seem rather obvious, you don’t want to use jars that aren’t made for canning, as they are more likely not to seal. Even though it is cheaper to reuse glass jars from the grocery store, it is ultimately not worth it if you end up with a whole batch of unsealed jars. Canning jars are made to fit the lids, seals and rings just right, so it is better to spend the extra money to make sure your jars seal.