Birds eating onion tops
I do empathise with you on the wet weather we are having this summer it’s not good for us gardeners trying to grow crops without sunshine and so much rain. Onions like most other vegetables like sunlight and well drained soil and this could be the reason why they are not growing. The loss of your onion tops is almost certainly down to birds, you didn’t say if your onions tops were eaten soon after planting or recently. If they were eaten soon after planting your sets it was probably small birds looking for food in early spring, if it was later then I would say it is down to pigeons. Either way it’s the reason your sets have not grown, all plants need the green leaves for photosynthesis so it’s very doubtful if they will grow much bigger now. Onions should be almost ready for harvest now and should be drying out on the surface of the soil but there is not much chance of that at the moment.
Next year if you decide to grow sets again cut off the dry loose skin at the top of the set before planting this will deter the birds from pulling them out. I covered my onions in Stephanie’s Kitchen Garden with fleece to start with until the onions grew and pushed the fleece off. You can then cover with net to stop the pigeons eating your precious onions.