Certified GKH Gardening Expert
Daphne can be just a bit particular when it comes to environment. I would, first, be sure that the soil has plenty of time to dry out down to about 3 or 4 inches or so between watering. Yellowing leaves will be the first sign of overwatering, with a quick downward spiral to the death of the plant.
If you are sure that the soil has plenty of time to dry out between getting a drink, then you can begin to figure out deficiency symptoms.
Feeding with an all purpose, once per year, is often enough to keep them plenty happy.
This article will offer more information on the care of the plant: https://www.gardeningknowhow.com/ornamental/shrubs/winter-daphne/care-for-winter-daphne.htm