"Mike Reed" writes:

If the tenon might be the problem, then, cut a mortise-sized dado in
maybe a 3" length of stock and drag that along the length of the tenon
to see where there's interference.

I suppose you can glue up a scrap making sure it's the "official"
mortise dimensions. Might be easier than cutting a dado, if you can
wait for the glue to dry.

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