I flew to Cancun, and was stamped in Mexico for entry. When I returned to the US, at DFW, no entry stamp was placed in my passport. I'm a US citizen.

A few weeks later, I went to Mexico for a mission, and drove to the border. The Mexican border official would not let me enter Mexico, because I did not have a stamp showing I returned to the US. He told me that I had to go to the border office, which was closed at that hour.

I'm driving into Mexico again in a few days, still without a US re-entry stamp. What should I do? The rules seem very lax.