The responses so far have covered several aspects involved in flexing a ski boot. the primary action that flexes a boot is having your center of mass positioned correctly so that as you flex and extend you hover just forward of the boot sole center. this will cause "G" forces to push downward and forward on the boot shell.
Lou is correct: having your bindings and boot sole center positioned correctly is important. Check out a Marker "schizo" binding, you can move fore aft with that one out on the hill and find the sweet spot---might be an option--- just saying.
Binding delta plays into the mix, boot sole length.
boot board angle plays a part
boot forward lean is involved
the size of your calf muscles will also determine where you hover over the boot sole.
If you have slim calves and are in a boot that is more upright you will stand on your heels at transition---G forces will push your butt downward behind your heels and the boot will seem to stiff. Your dorsiflexors aren't strong enough to overcome the loading at 2 to 3 "G's.
If you have large calves and too much forward lean---upward movement (extension) will push the calf muscle back into the shell and load the tail of the ski at transition. Your knees will still be too far forward and the loading of the calf is not the direction the leg needs to move in to flex a boot---it will seem like you can't flex the boot.
So----what is the circumference of your calves at the top of the liner?
How much difference between the height of the binding toed piece and hell piece where the boot sits on the binding (delta angle). also boot sole length?
how much forward lean in the boot---put a carpenters square behind the boot heel and measure to the liner at the top of the shell.