How deep did you plant them and have they been watered through summer?

Onion sets need well manured ground, so get cracking
in October after visiting your local stables !
Then plant them in March 2" deep, as the birds think they are grass and pull them out. If they are 2" deep they get a chance to form strong roots to prevent this happening.
Better luck for 2014. 'Turbo' are the best for long term
keeping. 'Mammoth' are rotten by November, best
avoided, unless you want them for a show.

Good advice on manuring Diane (makes mental note to do that). I set mine in March with the roots below soil level with most of the bulb above. Kept them watered and those in full sun and lots of water grew. Had a few that did nothing much (am guessing soil was too dry). You can still eat small onions I suppose!

Onions from sets are best started in the greenhouse in individual cell trays February time. Then when early April comes they have a good root structure and will grow away fast when planted out.

Thats good Scrumpy - if growers have a greenhouse or
frame. I found sets were always better than growing from seed.