AS Mileage Plan number too short to be accepted by British Airways check-in system
I recently joined Mileage Plan and was assigned a 9-digit FF number. Yesterday, when checking in online for a BA flight I tried to add Mileage Plan the number but it didn't show up on my boarding pass. At the airport I asked the check-in agent to add the Mileage Plan number and reprint the boarding pass. Doing so, she kept receiving a message saying the number is short. The system accepted any arbitrary 12-digit number, but not a 9-digit one.
Any guess about what's the issue?