Must one repeat the ablution if a part of the elbow was not moistened during the wiping of the head, or is it sufficient to moisten it and complete the ablution?

If you find a part of your hands that the water did not reach when wiping the head, wash only that part, then repeat wiping your head to maintain the proper sequence, because sequential order is obligatory in ablution. However, if the continuity is broken, it is safer to repeat the entire ablution.