Is this your first set of Flow's or have you had them in the past? They do take some tweaking when getting them setup and then usually a bit more tweaking once you get out on the mountain with them to get them dialed in. Once you get them adjusted correctly you should only have to make minor adjustments with the strap ratchets as they break in a bit. The high back has - for lack of a better explanation - a front and a rear position that is used to center your boot on the binding. I'm guessing that the high back needs to be moved back to relieve pressure on your boot. You can see the adjustment starting at the 1:10 mark on the following video:
FLOW Snowboarding | Video
Be sure to have a #3
Phillips head screw driver - the screws are loc-tited in and are a bitch to get out.
The video does a nice job of explaining the full setup on a pair of Flow bindings so you may want to watch the whole thing if this is your first pair of NX2's.
There are others here that are better with the technical side of the Flow adjustment and I'm sure they'll check in if there are other things to look at.