Hi,
You could try to pull the walls apart, I assume the end of the walls "the dot when selected" connect?
And then extend them to join again (force a-rejoin) (or use the corner join tool to extend)
Or it's more likely that in the original model a 3th wall is interfering with the join inspect all walls around the join.
Also to check, are the ends of the walls "allowed to join", copying/pasting the walls could force a join if only one end has disallow end enabled..
Good luck.
- Michel