Hi ttocs and thanks for replying! Yes we looked insid to figure it out and there is no piece of anything there and the depth is enough to go all the way in like the others. It really looks like it's just the threading that stops too early to continue to screw.hmmmmmm, . . . . .
Is it possible that some portion of the plastic shipping foot is broken off and stuck in there? A skinny screwdriver or pick could confirm if the depth of the pocket matches the others.