ONION SETS, AUTUMN OR SPRING SOWN?

We have spring planted heat treated onion sets for a few years. Two years ago we decided, due to the amount of garden activities in spring, we would try autumn planted heat treated sets.  The trouble is our autumn ones bolted a little last year and lots this, resulting in losing most of our crop.  However all our neighbouring plots who did the spring planted sets have got a really horrible white fungus growing underneath their sets.  I know the bolting was due to us not watering during a particulary prolonged dry period, so perhaps we could make a note not to do that again, but why are onions so difficult to grow this last couple of years, when they have been perfectly ok before that. ... not to mention the autumn grown shallots/garlic that also, either bolted or rotted!

I have sets sown this spring and I have only just noticed one onion that has mold on it - all the others are great,  none have bolted either but I will pull mine now to stop any more mold.
I am thinking of using seed and planting this autumn too - I think its about experimenting.
 For me opnions and peas have been terrific this year - not so cauliflowers - very disappointing. poor germination of beetroot, poor potato crop. Hope beans will be good