Use a set of calipers to mic the width and thickness of the tenon to see if
it is consistent. If it gets thicker toward the shoulder you can reduce the
thickness or width. If it the same all the way across the tenon the mortise
is to small at the bottom or the tenon is too long.
max

In article .com,
"nireedmot" writes:
most of the way into the mortised leg and then stops.
Is there a trick for how to tell what the problem is?
How can you tell exactly where along the 17" of joint
the problem is when nothing's visible?

Its the part of the tenon that gets slightly squished and
maybe shiny. You'll have to look *real* close. Is it
bottoming out or just getting stuck? Try taking the edge
off the edges, too.

Chris