Hi Xinran! A couple of things could cause this: 1) Make sure you don't have different base offsets for your wall or different top or bottom offsets or any other constraint that might misalign he top or bottom part of the wall. (you can find all those settings by selecting the wall and going to the properties tab) 2) Another thing that might cause this is the use of different phase settings for your walls. For instance if some walls are on the 'existing'-phase and others are on the 'new construction'-phase. They might not join properly. Good luck!