moving dynamics isn't always possible

This time I'm on MuS 3 and I cannot manage to move dynamics around.
Look for instance at this example of four measures.
While I manage to keep the fff at bar #2 of Soprano above the stave, there is no way that I could find to get the companion in the Alto to do the same. After I move it with the mouse it goes back to below the stave.
There must be a trick to achieve that, isn't it?
Thanks.