I've always had difficulty with large bulb onions.
My best guess is I've got too much compost and woody material in the mix. Assuming this leaves little soil for the onions to grow in.
This years are as sickly as any I've tried.
Everything else I grow does fine. I do add good leaf mold and aged manure to the holes for peppers and tomatoes.
Do onions need better soil conditions than other plants?
At this point, I am about to sift some of the woody stuff out of the onion rows and replace with what is left of my leaf mold, aged manure, and clay, then replant the onions that have not died.
Any suggestions?
I don't know the answer to your problem, as I've never grown onions, myself. I seem to recall that my late husband used to mix well-rotted sawdust into the soil of his onion bed. I'm really not sure of that, though. It was a long time ago. Here are links to some fairly informative pages about onions. Perhaps you can find some answers there?
https://plantanswers.tamu.edu/publications/onions/onions.html
https://ohioline.osu.edu/hyg-fact/1000/1616.html
https://plantanswers.tamu.edu/publications/onions/onions.html
https://ohioline.osu.edu/hyg-fact/1000/1616.html