Ran into this on my flathead some time back...the heli-coil had the end (down in the hole) angled across the diameter of the coil to facilitate inserting and was supposed to be broken off after inserting but had not been, so it did not allow the stud to go to full depth. May not be your problem but I never felt very confident with the setup thou I did finally get good torqe on the head at that stud.
