One motor stops spinnig at takeoff

Hi folks !
I’ve had that copter for a while but never really finished putting it together - feels like there was always something …

This time I feel pretty close! As I start taking of either of the front motor will stop spinning. I made a quick video to illustrate the issue. It’s here:

Any idea what may be wrong !?

hi i think i have the same problem how have you solved you problem ?

It looks like the orientation of the flight controller is back to front.

Simple test.
remove props
arm and put throttle up a bit in stabilise
tilt the copter in each direction and see if the low motors speed up and the high motors slow down.

Also check your RC transmitter the same way
push forward on the pitch and check the rear motors speed up and front slow down
same for all other directions.

thanks for answering.

I just change the MOT_SPIN_ARM 50 (slow rotation) and all is ok now my APM fly