Hello mharsma. The bigest reason for a diesel not starting is battery matinace. Your battery could have an internal short. If it has a short, even with a good jumper battery it may not start. Disconect the ground cable from the battery, and try to jump it. With the ground strap disconected, the battery can't short out. If it starts that's your problem. If it doesn't start then you might have a bad conection, on the ground, to the tractor, engine. It could also be bad battery post conection, check and clean both.
David-Davies