I've experienced this. It happens when you are trying to move a section directly inside the Navigator panel. Basically, if you look at the image above... If I take the "Section" already highlighted and tried to move it below the next "Section" (not inside of it) it can prove to be difficult if done inside the Navigator panel. Especially if the "Section" has a few things inside of it.
If you grab it then move it outside on to the stage, you can place it fine. That's my experience with it over the past three weeks since I've started. I would give the option of making them collapsible. Default could be to stay open.. but give the user the ability to collapse a main section if they would like to do so.
If you can't see it happening for you. I will make a video to demonstrate during my free time.