You'd likely get better results by hard pruning them.

Perle d'Azur is a type III hard prune every year and Niobe is a II that can be done as a III.

I would probably dig them up, dig a deeper hole, ammend the hole with manure/alfalfa, plant the crown about 4-6 inches below the surface of the soil, then cut the canes back to about 6-10 inches above the surface of the soil.

Robert

If you don't want to dig them up, just mound soil up around a large area around the crowns and you will in essence have planted them deeper. Cover the area with mulch and it will help the soil stay in place.

I also agree that if you have not pruned them back hard yet I would do that. I treat Niobe as a type III clematis in my garden.