Greetings Mary:
Well, there could be several logical reasons for this phenomenon, so let me try to briefly explain a few potential scenarios:
1) Some oak trees simply do not produce heavy acorn crops every year and some oak trees simply are genetically poor producers of acorns.
2) If your oak is in the red oak family, then you can typically expect heavier acorn crops every 3-5 years.
3) If your oak is in the white oak family, then you can typically expect heavier acorn crops every 4-7 years
4) Environmental conditions (e.g., heavy spring rains, growing season flood events, drought, unusually high/low temperatures shifts, etc.) can cause poor acorn pollination, acorn crop abortion, and complete acorn crop failures.
5) Early season frost can kill/damage oak flowers resulting in lack of pollination.
6) Pest or pathogens, confounded by weather patterns or events, may also be responsible.
Unfortunately, I cannot provide you a more definitive answer; however, I'm pretty confident that one (or a combination) of these factors has ultimately caused the lack of acorn production in your area(s).
