Check that the wall reference lines are actually touching each other.
If they are then check the 'Legacy' settings in the Options menu.
If the Legacy is already off, then it is probably an issue with Building Material strengths or more likely the walls are in different layers and each layer has a different intersection priority number (or it is set to zero if they are in the same layer) - this will prevent them from trimming.
One of the forum moderators.
Versions 6.5 to 25
Dell XPS- i7-6700 @ 3.4Ghz, 16GB ram, GeForce GTX 960 (2GB), Windows 10
Dell Precision 3510 - i7 6820HQ @ 2.70GHz, 16GB RAM, AMD FirePro W5130M, Windows 10