i find using sets is better than sowing seed. The sets are treated to help prevent flowering.

keep the onions evenly watered as if they get too dry they can can bolt ie flower.

If my memory serves me well, didn't you say that you grew onion sets in containers. Herein is the problem. Onions are deep feeders and I would say that best grown in the ground rather than in containers where the compost can either be too dry or too wet. If the sets are too dry then they will try to reproduce, hence running to seed. Best grown direct in the ground; and tight spacing so as to thin out using the thinnings early and allowing the remaining onions to grow bigger without competition.

Thanks both of you. I have always grown them in sets.
I did try seed once but to no avail. And yes, I do grow them in containers but I have followed the instructions for container growing. I am sorry to say there is not enough room for me to grow in the ground here. I seem to recall that I have produced them without flowers at some point in the past. It does not make any difference to the taste as far as I can see but I would just like to know!