Hi ygirouard,
not sure what's going wrong on your side, but drag'n'drop works perfectly here (also 9.1).
But yes, I can at least confirm, it's sometimes a little bit tricky.
If you are using the drag'n'drop method, there will be light gray line and if you let go your mouse button, it will be inserted at that position. If there is for example another folder on top (like in the screenshot below) and you wait a few seconds, it opens automatically and you can place it within this folder then. So if you want to place the assertion between two folders you have to be quick, otherwise you will always "jump" into the other folder.
Hope that helps a little bit.
Ciao Stefan