I'm not exactly an expert on this but have a similar problem on one of mine. If the screw won't really go into the the bearings at all, then it has to be a machining tolerance issue - bad alignment between the pair of bearings would make it go in half way and then baulk - and could likely be solved by slackening off the retaining plate on the BK and wiggling. Some gentle sanding might be the cure, alternatively would putting the screw in the freezer and the BK in a warm (50deg) oven enable it to slip over I wonder??? - it would be on for good then I guess so best to get the alignment right.