Good page about Onions I just found with Google:

http://bit.ly/lfJtqe

If they complete seedheads, there will be no bulb. All the plants efforts go into producing seed. Most folks will remove the flower buds as soon as they show, which slows down the process. Not a sure cure as the bulbs will be hollow and deteriorate rapidly. Usually pulled for green onions.

Thanks for your response. I'm still wondering what I can do to if I don't want to pull for green onions. How do you get the bulb larger and not hollow?