I'm still quite green with Eagle, but:
I think you need to have the jumpers defined in the schematic, in order for the board view to determine where the resulting tracks join and the footprint (device selected in the schematic view) allows sufficient room to span an area set out in the DRC.
If you switch back to the schematic view, delete a wire link and place a jumper, then reconnect the wires to each end of the jumper & then switch back to the board, I think you would then have the jumper available to move anywhere along the wire (track) as you wish.