Notes are not 'moving' any longer or cannot be picked up to be dragged and dropped, when their positions are not what they seem. This may happen when you zoomed the screen and the scroll bars are no longer on the left or the top of the screen.
As a side-effect, the Notes may appear double.
Notes may lose their apparent position when the screen is not refreshed properly and timely. When this happens, slide back the scroll bars on the horizontal axis all the way to the left and on the vertical axis to the top. You may also zoom the window to 100%.