Had it happen exactly as you describe with two new pipes, both good specimens of a top brand.
The first time it did drove me nuts, and I tried to "out stubborn" it. I also wanted to make double-damn sure it was the pipe and not something I was doing differently or wrong. Kept at it for several days.
Nope. She no work.
Since it was a rare(ish) specimen that had taken some effort to find, I didn't want to flip it, but I didn't want it as simple a rack ornament, either.
So, I decided to Rick Newcome it. Enlarge the airway diameter.
180-degree reversal. Smoked great, and has ever since.
That happened about 6-7 years ago.
About six months ago was occurrence #2.
This time I only smoked the pipe enough to confirm it was another Mystery Won't Stay Lit specimen, and went straight to the same fix, which was again the ticket 100%
What the hell is going on is something I doubt anyone will ever know. Some sort of freaky physics phenomenon like airway length to diameter to angle combinatorial ratio that creates the smoke equivalent of a standing wave resonance, and "breaking" the ratio by a small amount is all it takes for things to flow normally again.
Or something.
Whatever it is, it's absolutely real.
Also nothing any carver or company can see coming. (It's not a quality issue)