I know that many experts will probably disagree, but here goes:
Some years ago, I asked the (sadly no longer with us) very nice septuagenarian gentleman next door, for advice about growing onions, and he told me two things:
1) It can take some years to get an onion bed really up to scratch; no rotation for him.
2) Onions love company, so plant them in small groups.
I have since discovered vaminoc mycorrhizal fungus, which helps with both yield and disease inocculation, and haven't really looked back.
So, you're probably doing little wrong (otherwise none would grow), and perhaps experience, patience and time will reward you.
The manure may be too fresh and thus too strong?