I have to wonder if all that excess of soil amendments might be what is attracting underground critters to eat your seed.
My father in law's family ran a truck farm in Ct 50 years ago and they raised onions (among lots of other veg).
The soil where onions grow has to be tight enough to support the top weight of onions green growth. Silty clay and sand he said were the best conditions for their root crops.

I have no luck sowing in site - sowing in seed flats and then transplanting is what works best for me. I know it sounds strange to say that those tiny green wisps will grow into full sized onions but they do. A problem I have with growing onions over the winter is that most of those bulbs will try and bloom next Spring/early Summer and blooming onions don't store well and I like to have onions in storage all year. Spring sown onions don't try and bloom so they store just fine.

I've grown onions from seed for over 10 years now, sowing in a cold frame in October, and setting out in March, and my onions rarely bolt.

Gosh!! I am so behind in planting !!

So... I just got some sweet Onion sets and am planning to plant them Saturday ( tomorrow)

Is it early, late or OK?

This is my first time with onions. I am pitting them in raised beds. Any suggestions ? Thanks!

I think you'll be fine with sets. My seedlings are a bit behind so I haven't even started to harden them off. Just try to get them in the ground and growing before the weather gets hot.

Thanks !! John.