I'm still curious to know if you are able to inflate the tire. Sorry if you've answered that previously. But it does sounds like if you attach the chuck properly, the pump and gauge work fine. Is that correct? If that's right, you found the source of your problem.

Its not unusual for an O-ring to wear out after hundreds of times pushing it on and off the stem.

The problem I see from looking at my Lezyne Flip-Chuck, I can't see an easy way to remove the O-ring on the Presta side of the chuck. (My chuck is reversible.). If you have the same kind of chuck, you may have to use something like a dental pick or sewing needle to pull out the worn O-ring.

If it is gonna take a while for you to hear back from Lezyne, or its going to cost a ridiculous amount of money for a 45 cent O-ring, you could take the O-ring to your local Ace Hardware store (not a Home Depot type store) and match it up. O-rings come in standard industry sizes and it is pretty rare for a company to have a special mold made for a proprietary O-ring size.

This may sound a bit unsanitary, but whenever I put a pump chuck on a valve stem, I always lubricate the stem first with a handy lubricant I always have with me - spit.
​​​ This helps keep the O-ring from wearing out when it's pushed past the screw threads on the valve stem. But it will eventually wear out. Thanks to you, I now know how to change it!

My guess is that Lezyne will come through for you and send you the O-rings you need gratis.