There are several things that can prevent a jar from sealing:
1. Headspace too big or too small (headspace should be ¼ inch)
2. A chip on the jar rim
3. Using an old lid (new lids must be used)
4. Screwing the band on too tightly (band should be fingertip tight)
5. Jam on the jar rim (rims should be wiped clean)
Click here for our Step-by-Step Tutorial on Water Bath Canning.