Might not be the case but last year I had similar problems. Onions and Garlic grew to about 5 inches above ground then stayed like that. The onions were loose in the soil and the garlic wasn't getti ...
I've had lots of DDLs in my polytunnel too, with the last 4 weeks it's been like a DDL brothel. Are they harmful? They don't seem to be damaging anything.