I just put together my first quad using the ZMR250 kit with an XRacer F303 flight controller.  Plugged it into the computer and opened up cleanflight, calibrated the accelerometer, tested motors (all spin up), then looked at the receiver settings and made sure all the controls on my stick map properly. Everything looks good.

But with the USB disconnected, I can't get the transmitter (Turnigy 9x) to spin up the motors or do anything at all. As far as I understand it, my connections from the receiver to the FC are good since the data reads correctly in Cleanflight. And the connections from the FC to the ESCs are good too, since they spin up when testing motors in Cleanflight. So what could be the issue? I tried arming it by moving my throttle stick (mode two) down and to the right, but nothing happens. Help!

Check your throttle is set below 1000 in Receiver Tab in Cleanflight, use trim switches to raise or lower the throttle setting.. Try that first.

Thanks Biggles, tried that earlier and nothing. I just fixed it by flashing the latest Cleanflight online firmware... for some reason arming with the latest Betaflight wasn't working properly. Standard arming/disarming procedure working now!