Every once in a while, the spindle/knuckle will have a bad place in the threads where the caliper mounting bolts go into them. Had to get a tap and clean them out when I last changed the front pads on my Burb, cause the passenger side bolts wouldn't go all the way in. First time I have seen this, after many GM and Mopar disc brake servicings, but proves it can happen. IIRC it's an 11mm thread, take the bolt to match thread pitch.
Not saying that this is the issue here, more likely aftermarket calipers, but just putting it out there....