cover them with fleece until they are firmly rooted in was what I did originally. I found it was birds pulling on the papery shallot skins, looking for worms etc underneath but once rooted in they gave up.

The squirrels didn't seem to bother with any of the onion type sets.

Did you cultivate the soil before planting them, or did you just push them into the soil?

I know it sounds odd, but they could have popped back out before taking root if the soil wasn’t tucked in tightly enough when planting.

I did push them in a bit more than usual.
They are covered in fleece now thanks to Seaburngirl.