Great job swapping the zones which rules out an issue with the controller. The next things I would check include (not in any other):
- Checking all connections for the bad zone including the terminals on the controller and the wires from the valve going into their wire nuts (they do work loose more often that people want to admit)
- If you have a multimeter either:
a. check voltage on both ends of the wire when the controller has the zone on
b. check the ohm on the controller end with the zone off - Manually turn on the valve and see if it turns on, then turn it off
It would seem to be something with the wiring or valve. We are trying to narrow it down. If you have more questions, need more details, have a report, or anything, please let us know.