Sunflower seeds

Mango-grower starts with ...
I have several large Russian Giant sunflower plants. I picked one that looked like the seeds were ready yesterday and separated the seeds out, but most of the seed husks have no seeds in them. About 10% have seeds. Is this a pollination problem? I put a stocking over the flower to keep seed-eating insects out after the petals started withering, perhaps I did this too early? Do bees pollinate sunflowers? We don't see many bees here. I live in western Brisbane.

Thanks.