Q: What would cause a large group of winterberry plants not to produce berries?
A: The most likely cause would be no male pollinator.
If you've got all female winterberries and no male anywhere nearby, that would give you healthy plants but no fruits.
The same would be true if you have all males and no females. Only the females produce fruit and only when pollinated by a suitable male.
Another explanation would be you've got males but ones that flower out of sync with the females. Not only do you need nearby males to throw off pollen to produce fruit on the females, the two plants need to be blooming at the same time. If the males and females are blooming at different times, the deed won't get done.
That's why it's important to pick the right males and females at buying time. The females 'Berry Nice,' 'Red Sprite' and 'Maryland Beauty,' for example, are pollinated nicely by the male 'Jim Dandy.' However, 'Southern Gentleman' does a better job of pollinating 'Winter Red.'
A third possibility is a lack of pollinating insects to move the pollen around. Spraying insecticides is a leading reason for that problem.
And fourth is some sort of seasonal stress that aborts the whole process. This year, a possibility is all the excess rain and cloudiness at pollinating time. Disease is another stressor. And so is intense heat and drought that can cause young, pollinated fruits to drop before they mature and turn red. This one is typically the cause when you've had nice fruits in the past but suddenly don't get many (or any) one year.