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.