When it comes to new plants from cuttings the best time to transplant is when the weather is cool such as early spring. Cooler weather promotes root growth over top growth. The other clue to what may be the problem is a full sun area. Viburnums actually prefer shade from the hot afternoon sun and as a broad leaf evergreen the new cuttings will wilt in full sun. Try shading them with newspaper tents for a few weeks if you must plant in full sun. Add a 2 inch bark mulch to keep the soil moist. You might also try growing the cuttings in one gallon pots for a year after the cuttings are rooted. This gives them a better root system so they can handle the transplant. Don't give up, growing plants from cuttings is a fun and money saving skill that can be learned and even enjoyed with some practice. You are learning from your failures. Keep Growing, Marianne Binetti