ESC/Motor Won't Start Until 50% Throttle

I’m having an odd issue with my octocopter. I flew it yesterday morning and it worked fine, but then two arms mysteriously stopped spinning when I tried to fly again in the afternoon. Two arms right next to each other wouldn’t start up, but the other six would.

I thought perhaps the ESC’s had given out, but when I removed the props and tested the full range of throttle in stabilize, it revealed that the two arms would begin spinning at 50% throttle, but would not start spinning unless it was at 50% throttle or greater. I re-calibrated the ESC’s through Mission Planner, but that did not change the results. Any suggestions as to what to try next?

Is the problem solved? I have the same problem…