Esc calibration problems\issues

So… I’ve been building a hex copter from scratch and having a blast with the build… Until now. It is in its final stages and I’m setting up my ardupilot flight controller. Unfortunately it is an older board and cannot be updated anymore. My problem is this. I calibrated all 6 esc’s and motor direction. All 6 works perfectly. Then I connect my flight controller and set up on mission planner. All but 1 motor spins. And when dead sticked that one motor that isn’t turning just beeps. A single beep over and over as if it isn’t calibrated. I have calibrated it at least 5 times now but for some reason it won’t work. I am unable to use the mission planner esc calibration due to my older version. But the motor test works but still have the same issue with motor 2.