Onions and shallots failing - any idea why?

They were thriving happy onions, and then they started to go down.    Now they are like this - pics below - and the shallots (about 3 feet away) are going the same way.  The runts and spares planted elsewhere are growing fine, so I don't think it can be an infection that came in with the bulbs - which came from 2 different sources anyway.  There are no maggots at the base.  When I cut one of the failed bulbs in half it was just rotting from the top (sorry, didn't think to photograph that).  There were potatoes here last year, so I am rotating the crops - it has been years since there were onions here.  The ground was manured with well rotted cow manure ...  the chinese cabbage and pak choi between the onions and the shallots are thriving so it isn't aminopyralid.

Any ideas?

I've got a couple gone like that. I planted them in the garden away from the others, because I did not have any other space for them, and they were doing well at first, then they started to deteriorate over 2 or 3 weeks.

You are right - first leaf sliced and 2 maggots found.      I thought that onion fly maggots were found under the bulb.  Every day a school day I suppose.  I've been looking up the life cycle and crying gently - bye bye onions and shallots for this year.  This afternoon I shall clear the lot in the hope that the garlic aren't yet infested.  Serious boohoo ....

Thinking about it, our plot neighbour had to give up mid last year with a back operation and it wasn't till May time that she accepted that it wasn't going to get right in a hurry and resigned her plot.  Can't blame her for a bad back but if she had even some flies and chances are that they will be around on allotments, then they would have gone full cycle .....  and the buzz to us very short.  And it was about mid may they started to show odd problems ....

Oh *"£$%^&*()!!!!!!

The maggots may not have reached the bulb yet. Try cutting open one of the bent over leaves to see if there is a little white onion fly maggot in there. They live in the leaves and ten travel on down to the bulb to pupate

RHS advice says

"Onion fly can be distinguished from allium leaf-miner and leek moth as it feeds on roots rather then within the leaves."

So if you find grubs feeding in the leaves it is not onion fly.

The next thing you have to do is look at the grubs and see if they are maggots ( leaf miner) or caterpillars (leek moth. Maggots have no head or any legs. Caterpillars have a head and legs.

https://www.rhs.org.uk/advice/profile?pid=885

I get allium leaf miner now as well - never used to. More crops to cover - so frustrating!!

And even if there as a chemical to get rid of these pests, I would still use the nets as then yes, I know what I am eating too, jaydig!!

I have sacrificed the onions and shallots

That's a real shame, but not much else you could do    Fingers crossed your garlic is OK.

What about lettuce or other salad leaves in the place of the onions?  If you stuck to quick cropping veg, you could keep the ground turning over and hopefully destroy as many of the horrible little beasties as possible.