Originally Posted by Ted Mittelstaedt

When I first started canning seriously a few years ago I ran into this...
This is what
I found out:

...

3) There are 5 failure modes on a fail-to-seal lid:

a) Too much pressure was used, and the jar glass pushed
all the way down through the sealing material and contacted
the steel of the lid. If you heat the lid and when it is hot, you
use a lot of force to really ream down on tightening the ring,
you can cause this to happen.

b) There was a chip in the glass jar sealing surface.

c) You used a new lid with a narrow sealing surface on an
old jar with a wide sealing surface, and the lid was not centered.

d) There was uneven pressure applied to the lid by the ring.

e) The sealing material on the lid had a gap or was too thin,
due to a manufacturing defect.

On the vast majority of fail-to-seals, it was the uneven pressure
problem. We can all screw down a jar lid. But, getting it
evenly screwed down, that's the trick.

...

Ted