I doubt that every animal she has is not breeding sound, I know a couple of them came from me and had been fine. Assuming these are not all related that would make 4 different chins that are not breeding sound out of the small number she has, doubtful.
It's possibly that if they still born came from the mom who died after birth that there was a toxicity issue.
You have a Murdoch's there right? What brands of rabbit feeds do they sell? Do they offer Nutrena? The Nutrena site says they sell it at Murdoch's, it's one of the brands that Gene Adcock recommended to me once.
I would also see about doing a gentle antibiotic as a preventative, you need to always make sure you follow through and don't extend treatment though.