Onions won't form bulbs

Hi All:

Last fall, I planted some Spanish onions that were supposed to overwinter and be ready this spring. Well the the green shoots got really large and formed nice seed heads but they never formed a bulb. Just a long thick brown end where the onion bulb would be. All my other onions did well.

Any ideas on what may be wrong?

BB

All the Spanish type onions that I know are long day onions. Need short day onions for overwintering.