Viburnums often suffer from leaf blight and leaf spots, and this is a fungal infection encouraged by wet weather and fungus-filled soil. You treated it with an insecticide that does not kill the fungus. The good news is your viburnums should recover, and the damaged leaves should fall off. I suggest you add a mulch of dry wood chips of beauty bark this summer. The fresh, dry mulch will keep the fungal spores in the compost you added from splashing up onto the leaves next spring.  In this case, spraying a fungicide would not cure the disease, so don't waste your money. Ensure you did not pile the new soil up around the stems or trunks of the shrubs. Shrubs that are stressed from too much soil around the trunk are more susceptible to disease. Expect a better display of foliage and flowers next spring and Keep Growing, Marianne Binetti