Really scraping the barrel here (I am intrigued what could be causing this!), is the QR bar bent in any way? I’m wondering if the first 4″ is taking the pressure off the dog?
Something is not right with the QR spring / bar, it shouldn’t be that hard to turn it either? Maybe whatever that problem is is not letting the dog fully engage on the drive screw? Is the QR lever easier to move when the vise is only 4″ open (rather than fully open), i.e. not under as much tension?