Most common reasons are hdop and compass disparity. Best way to find the answer is to connect to MP while arming. MP will flick up a brief message as to what the fault is. I suggest no props if using a USB cable.
Hope this is helpful.
Try the throttle trim on the transmitter 1st , if not you might try a throttle calibration or check if there's any power running to the motors.
Is it set to spin motors on arming? Have you checked the throttle received from the Rx? Does this also do this in Arco mode, Acro mode ignores many inputs and will arm and operate without checking for HDOP and other things.
Seem odd that loiter will arm indoors!
Also try calibrating ESCs and check the throttle isn't reversed on the Tx.