This year my sycamore tree stopped producing itchy balls. This tree produced itchy balls ever year we have lived here -- 40 years. Why did it stop?
There may have been environmental issues with the flowering and seed formation of the tree.
There can be variations in the amount of seed pods formed each year and your tree will likely return to seed production next season.
You could check with your County Extension Office to see if other reports of lack of seeds have come in from your region.
They may be able to give you some help on weather effects on your tree.
Here is a link to help you locate your nearest office.