That's coming along nicely! If I'm seeing this correctly the problem is that you have one of the freehand stems that has a ball right after the tenon and the ball is hitting the shoulder of the plateaux so the tenon can't go in...If that's the problem, get a dremel and open up the area so the ball has clearance and the tenon can slide further in. You might want to radius that sharp upper edge of the mortise- kinda prone to cracking when it's that thin...