A strange condition regarding arming and firmware

Hello, dears,

I met a strange condition in Mission Planner (MP).

My platforms:
ArduCopter (AC) Hexa, APM 2.5.2 and 2.6, MP 1.2.98, firmware 3.0.1.
I can arm my AC with the aforementioned platforms.

When I upgraded my APMs to firmware 3.1, 3.1.1, and 3.1.2, I failed to arm my AC in MP.
After I erased EEPROM, I re-installed firmware 3.1, 3.1.1, and 3.1.2 and completed necessary calibration (i.e., Live, Compassmot procedure, Radio, Accel).
However, I stilled failed to arm my AC in MP.

By the way, if I downgraded the firmware to 2.9.x, I cannot perform Accel calibration in MP.

Does anyone have suggestions?
I appreciated your help.

please try running the wizard, for setup, as this covers all the required options.