Had my nephews plant some onions sets last year and they seamed to do quite well but having spotted a few going to seed i decided to pull them up. Well i have a load of very small onions noting to shout about just have to decide what to do with them some are the sized of picked onions.
cam
the the answer is simple your sets are problem
I made the mistake last year of thinking my onions can look after themselves, but they are shallow rooted so they soon got too dry and bolted, I can see the same problem in a grow bag.
No sets this year, growing from seeds, and they get planted a bit deeper nearer the moisture.
Yes though that might have been the problem although i water into a funnel in each bag every night not sure that was enough in this heat.
I have 3 nets of onion sets this year in soil the grow bags were a one off :) just need to do a lot of pealing tomorrow :) going to blanch them and freeze them as they are and use in casserole :) hope that works :)