I found this problem often but i can't understand why sometime it works as expected and sometime no.
Opening the prioritize windows from various areas (Hit, List, Calendar etc...) i get the placement you see in the below picture.
In that case i can't move the windows that remain fixed as you see, the only way to proceed is to close it with ESC key and exit Weekplan. Sometime it works running again WP sometime i have the same results. This picture comes from APP , but i had the same problem sometime also in the browser (Firefox)
Customer support service by UserEcho