I don't want to sound like a dick but are you over tightening them? I find I have to tighten my boots about 75% of the way in the morning. Wait until the blood gets circulating well for a few runs. Then I cinch them down tighter.
There's a tradeoff with boot size, you want them to be pretty much as tight as you can get without being uncomfortable or cutting off circulation. A good fitting boot you don't have to tighten as much as a poor fit.
Also, how tight are you wrenching on your straps? Do the bindings fit the boots or are they too big?
Just want to rule out all problems before you have to fork out money.
This is good advice, I tried it last winter every time, looser, really loose, tight, not too tight, would also vary that on my straps too, nothing would matter, about 30-40 minutes in, both feet are equally numb...the only thing that I have had that has not changed are my boots. That is why I am thinking of those newer burton's.
I have a normal heel, but a flat/wide front/toe area though.