Docking issues - won’t move to the line

I’ve learned to use the docking tools an they are very helpful. I’ve run into an issue however with the attached file where I can’t seem to dock the objects. I want to dock the smaller rectangles to the larger rectangle but when I try and Dock Right or Dock Left- this is as far as they move. I am able to cursor them over to the box where I then weld them in to make finger joints but I can’t dock them. I tried putting a new page thinking maybe I had some hidden line or box but that didn’t work either. I don’t see any logic to why sometimes it doesn’t dock and other times it does. Thought I’d reach out for some answers. Thanks in advance.