Sounds as if you have given them to much Nitrogen in their feed.
But why do you want bulbs?
The general idea of Spring/salad onions is to produce long slim stems with
a good green top.
In my days of market gardening you would get complaints from the wholesalers
if your onions were bulbing up.

--
David Hill
Abacus nurseries
www.abacus-nurseries.co.uk