Might be the fertilizer. Other than a top dressing of well aged manure, which also acts as a mulch, I never fertilize anything growing in the yard. I do fertilize trees and shrubs that I grow in containers because they can't get nutrients on their own. Most 'experts' say that fertilizing anything growing in the ground is a complete waste. Grass is the exception. Trees and shrubs do get some of the those nutrients when roots are within range of those grassy areas.

Come to think of it, our 'Mariesi' and 'Shasta' are in areas that get no mulch or nutrient of any kind and they bloom heavily every year spring.

Interesting points. I'd definitely consider that.

Mine bloom like crazy with no fertilizer whatsoever, and they are 12-15 years old. They're planted over old roadbed, too, so not getting much from the soil!

There's a garden saying--new plants sleep, then creep, then leap. That is probably what is happening with your viburnum. The first year it came from the nursery where they probably fertilized it heavily so it bloomed when you planted it. Now it's busy sending out roots and leaves to help it grow. Flowers come after that process with many plants.

Assuming it's planted with decent soil, and enough water and sun, you should see better results in the coming years. And I do fertilize shrubs and trees the first few years and occasionally thereafter--tho I have not fertilized my Shasta in many years and it is covered with flowers every year. Planted 1996.