Walls stop joining

This has happened numerous times over the years. workign on archtiectural
drawings - laying out walls etc. They typically automatically join/clean up
as expected. However, every so often in a drawing that has been developed
and modified over time, they just stop. They basically just overalay each
other without joining. Also, if you add a door into the wall, they stop
automatically snapping and joining into the wall...again, they just lay on
top of the wall. Anyone have any hints?

thanks,

Larry