Thanks everyone.
I almost gave up and contacted Matt about replacing the unit, but I had one last try inserting the RAM, angling the SODIMM right next to the outside of the retention clips and pushing down until the Ten64 board flexed. It finally booted with the module and still worked after unplugging and shaking the unit around.
So yes - if you see this issue, you may need to be fairly assertive when inserting the SODIMM.