Originally Posted by
BuildingMyBento
This almost happened to me a few months ago.
I entered Mexico through Reynosa; apparently, land borders are treated differently, which meant that my entry papers weren’t stamped.
When I tried boarding my Guadalajara flight to Reno, the gate agent told me to go landside to some “immigration booth” and get an airport stamp. The boarding doors would have closed, had it not been for a handful of others in the same predicament.
Unfortunately, I’m going to guess that you don’t have much of a shot with a refund.
This sounds like the airline was wrong if it didn't know that land entries don't result in passport stamps.
If getting the stamp at the airport really is required, airline/airport agents should be well aware of the issue and verify the stamp when they first inspect the passport at check in.
I wonder whether the OP's original flight was oversold.