Arthur,
The thought of mis-alignment occurred to me; but I don't think this is the cause. The reason being that 4 of the bolts stand a bit proud of the cover when inserted and engage the threads within a turn or two. The other 2 bolts don't stand off at all and rest firmly on the bottom of the recess within the cover; without engaging any threads. it is as if the threaded hole is deeper, which would be the case if the top few threads are stripped.
regards,
Mark