I had this issue a fair bit when i was first sorting out the startup gcode for simplify3d.
That and heating up but never starting printing.

basically you need to change the startup gcode.

Assuming it works in makeerware, then that's most likely your problem.

One thing you might also try is the knp3d convertor: https://knp3d.com/software/#knp3dconverter

far as I know it's a pre-tweaked version of gpx. My knp works best when setup as a replicaotr dual, so hopefully might make a difference.

But try makerware first.