Pixhawk PX4 not arming

I recently purchased a pixhawk 4 kit and in the process of saving up to buy a tarot 680 pro kit with motors and ESCs, I come from a fairly well experienced backround of building 450 sized helis and have some experience with CX20 drones.

I have set the pixhawk flight controller and GPS on a timber board and bound it to my DX8 using the ppm module.
I run the initial setup on APM and can get the flight controller to arm. blue light flashes. the next day i try it again and then its back to the yellow light flashing and cannot arm. i run the initial setup again and it works.
I have replaced the micro sd card and this makes no difference.
I also cannot get a GPS fix.
I have also tried setting it up through mission planner, same problems

Please hel

@Pixhawk PX4 not arming,
Do you have a Pixhawk or a PX4 as the two navigation controllers are somewhat different?
Also, what version of ArduCopter are you using?