In general the later caves seem to have been painted on finished areas as excavating work continued elsewhere in the cave, as shown in caves 2 and 16 in particular.
According to Spink's account of the chronology of the caves, the abandonment of work in 478 after a brief busy period accounts for the absence of painting in places including cave 4 and the shrine of cave 17, the later being plastered in preparation for paintings that were never done.