We find that sometimes users will "Close" or Delete a Sales order and NOT deallocate the material, this then leaves the material stuck allocated to the order that has been closed or deleted.
My suggestion
If you click the "Close" option on a Sales Order, from the right, the system should automatically run the "Automatic deallocation" process.
If you delete a Sales Order (using the trash can), the system should automatically run the "Automatic deallocation" process.