this may sound a little silly..but are you sure you're seating the hex wrench in there all the way? I've had a few hex bolts where I didn't think the wrench would fit and it took a little force to get it to seat properly.
I know my hex wrench set has a few in between sizes (2.5 mm comes to mind) but i've never seen a size in between the 6,7 or 8.