Jar Seal Question

How are your jar lids indicating there is no seal to you? Are the lids not "popped" down?
That doesn't mean that they're not sealed. That means that there isn't a vacuum (or at least negative pressure) inside. No biggy. Give the rings a twist to further tighten if you'd like and should be fine.

Even though you tighten the lids down completely after you fill the jars doesn't mean that some won't loosen over time. I have ~40 cases of pints, and periodically I'll hear a loud "ping," but I don't investigate as I the answer is to periodically check all of them and re-tighten.
I favor anaerobic aging as it duplicates the aging of old tins, from which the taste experiences of aging are derived. Smokers say I opened a tin with X number of years and I tasted this. Using jars is a matter of storage than can become aging and is of more recent date. Whether a lid loosens and air re-enters the jar means only that the tobacco will change from anaerobic to aerobic fermentation, but how this changes the flavor is probably beyond the palate of anyone.
If you find that the tobacco has dried out, rehydrate. You probably won't be able to tell the difference between it and a jar whose lid did not loosen.

How do you get them to pop down and seal in the first place? Are you boiling them?

They're sealed when you screw the ring down tight. Sometimes, depending on how the tobacco is fermenting, they'll pop down, but if they don't it's not an indication anything is wrong.
Boiling them like you're canning vegetables is completely unnecessary.

My experience with the build-up of internal pressure.

If you screw the lid down tightly enough, internal pressure will cause it to bulge, putting pressure on the sealing rubber rim.

If you want anaerobic storage, you can vacuum seal the jars easily. I have an adapter that lets my vacuum sealer seal jars. I won't be using it for tobacco, since it appears that just putting the stuff in jars and cranking down the lid is best. But if you want... it is easy to do.

If I hear a random lid pop on a Ball/Kerr jar, I just assume that it is a change in temperature causing it to happen. My jars probably see a 15 deg. F swing between the winter and the summer, and once in a while, a lid makes noise. ::