I think we have a winner. There absolutely is a heel cup adjustment on the NX2 bindings. There a two holes on the base to mount the highback/heel cup pivot. And they ship in the forward position. It seems you had the right size the first time.
Flow NX2 Series "How To" on Vimeo
About 1:07 in it shows the heel cup adjustment.
this^^ good catch john doe,hopefully that would be it and he can enjoy the snow and flow....
2015 Rossi One Mag 159
2015 Capita Charlie Slasher 161