I always use the bolt release button to close the bolt (I also own SBE’s 1, 2 & 3 and know the importance of letting the bolt slam home), BUT following your advice, I experimented with the bolt and carrier and discovered just how sensitive the carrier is to bolt seating. For example, if you draw the bolt back to confirm that a shell is in the chamber before loading the mag, you’re probably not going to get the bolt fully seated again.
At least you gave me something to look for the next time it happens. Thanks