A dead spot in an armature will require either a new armature or a rewind of the old part. With all parts back in serviceable condition, the field has to be flashed to remagenitize the field pole shoes in the proper polarity of the battery. It will generate enough voltage on residual magnetism to pull the cut out relay pole to the on position to bring the generator on line while turning.
A dead spot could be an open between the commutator and the winding or it could be a short to ground due to insulation coating break down. Even one dead segment can keep the armature from proper function. I'm not one for motoring a generator. They are not designed to be a motor and can overheat rather quickly. If they overheat then they can throw solder from the commutator sections. If a person decides to motor a generator then just do it momentarily. There are other ways to test one.