Agree you are probably getting the flower too early. Ideally if the flower heads aren't brown and dried up good when harvesting You could leave a few in the ground to fully ripen than take the other onions for curing.
Kind of like I did with my garlic to get the bubils good and ripe. I cut the scapes off of all but a few. Only don't do what I did and get carried away harvesting and take them out as well.
