Most of my failures have either been from lids more than a year old or having forgotten (maybe) to wipe the jars before placing the lid.  I have been, in the past couple of years, paying more attention to the age of the lids after reading at numerous sites that it can be an issue.

I also wonder just how using lids which come with new jars might be a factor.  I've even had new jars ping in the car on the way home!  If I have to really pry open the jar, I usually discard that lid.

Bottom line is that I try to not get too far ahead on purchasing lids.  But we really have no control over how long they were stored in a very hot warehouse somewhere.

Since we are now sharing how we warm our lids  will add that I have thought very seriously about using that old potpourri pot for that purpose, especially on a day when I have a huge amount of canning to do.  However, so far I am still using a shallow but wide crockery bowl.  Put the lids in it (kinda spread/fanned out) then add hot water.  Usually I heat water up in an electric kettle to boiling.  By the time it is added to the lids in the bowl it is hot but no longer boiling.  And they are often sitting there long enough for the water to become tepid.  They still work because the sealing material has been softened, and that is all we are looking for.

Coincidently, a good friend shared today that she has quit buying Ball lids altogether, for this reason!  (I promise, I did nothing to get her to say that – she just volunteered the info!)