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You should at least be able to see a flower bud by now. Examine the top of one and see if you see damage. It is possible that a squirrel ate the flower bud off. If you don't see any damage, wait a bit longer. It may be just a slow starters.

Check some of the nurseries that sell organic and non GMO seed. Seed Saver's Exchange, Baker's Heirloom Seeds and Territorial Seed Company are some that immediately come to mind.

Here is an article or two that you may find helpful: https://www.gardeningknowhow.com/ornamental/flowers/sunflower/sunflower-problems.htm
https://www.gardeningknowhow.com/plant-problems/pests/keeping-ants-of-flowering-vines-vegetables-and-flowers.htm

This is puzzling. While birds could definitely be an issue, it sounds more like the Sunflower head-clipping weevil (yes, there is such a thing). The pest will make a ring of bites around the head until it finally falls to the ground. Since yours appears to hang like it's been clipped, I'll wager that's the problem. Here is some helpful information: http://www.entomology.k-state.edu/DesktopDefault.aspx?tabid=654

Here is an article that you may find helpful about additional pests: https://www.gardeningknowhow.com/ornamental/flowers/sunflower/sunflower-problems.htm

Sunflowers are a natural food for squirrels and many birds. You can try tying aluminum pie pans to the stalks to frighten them, but those little critters are smart and outwitting them can turn into a full time job. These articles might offer some other suggestions: https://www.gardeningknowhow.com/edible/fruits/fegen/protect-fruit-tree-birds.htm (some of these ideas might work on your sunflowers too)
https://www.gardeningknowhow.com/plant-problems/pests/animals/get-rid-squirrels.htm

Here is an article that you may find helpful: https://www.gardeningknowhow.com/ornamental/flowers/sunflower/sunflower-problems.htm