i can't change tasks order

hi,

i don't know why but i can't change the order of download of my tasks ,when i try to drag the task up or down i can't do it and my cursor is a ''forbidden'' icon , i tried to add the ''up'' button to the task bar but i can't click on it , so i would like to know how i can change the order of download of my tasks and if is a method to lock the position of the task list how i can disable it . i also tried ctrl+up keys but nothing worked , what can i do ? before ,everything worked fine but i don't know what's wrong .so if somebody can help me it would be great .im using windows 10 build 10162 x64. thanks