Seedless Sunflowers?

Hello everyone
My sunflowers produced very large flowers with lots of seeds. Unfortunately the seeds are empty (no seeds in the shells).Any clue why this is??.
Thank you
M.

 

Probably lack of summer heat meaning the flowers are produced so late in the season that the seeds can't develop.

 

This is just a guess but they might not have been pollinated enough to make complete seeds.