I had that problem too, tried everything I could think of and nearly asked during on of RD's live streams...
I had one last look and if you right click on a port you will see 'snap' which is on by default. You can turn that off and it will dock. I guess it's a feature so you can build whatever you want, line up all the docking ports and don't need to worry about manually lining up. I was just too stupid to notice