moving an effort to another task/subtask

I often decide to organize the efforts (work) that i've done for a project, by later dividing the project in smaller subtasks.
In this case, I need to re-classify certain works (efforts) to other tasks than the original task to wich they were recorded in taskcoach.

this means, I need to move a batch of existing efforts from one (sub)task to another.
But this does not work if I drag and drop the selected efforts from the effort-window to the wished new (sub)taks in the tasks-window. At the tip of the mouse-cursor I see a "forbidden" sign when I try to do this.

Thanks in advance!