I had the same problem three weeks ago. I then removed the eMMC, but I asked in the Armbian forum if this is normal. I didn’t remove the heat sink and think I could fiddle with the eMMC later.
The RockPi does not boot Armbian Stretch from the inserted SD card if an eMMC is installed at the same time. As soon as I remove the eMMC it works. An image of radxa boots even if eMMC is installed. This behavior irritates me a bit, is that normal?
probably not.. But you have at least to provide a bootlog from serial console to get a clue where it messes.. RockPi is in early stage of board bring up (wip). Bugs must be expected..