FBM listing won't convert to FBA - any suggestions before I give up?

I have a listing that won’t convert back to FBA - been selling on FBA for a long time, ran out of stock so switched back to FBM, as I usually do, until I the new stock arrived at the FC. It arrives, went into stranded inventory, but wouldn’t then switch back to FBA.

I did the 1-click change, and I did the more traditional change route to convert it, and I tried it via the SKU page route, but each time it looks to accept the change (please allow 15 minutes for it to convert message came up) but it never does switch.

Raised a case with SS - first response said it was an error in the Amazon system, now fixed and should convert OK (but didn’t), re-opened the case - 2nd response was an answer to a random question that wasn’t asked (good old SS), then I went onto SS chat who confirmed seemed to be a problem their end, would look into it, then 24 hours later got a follow up saying there’s no problem as it’s already switched to FBA (which is isn’t). It absolutely won’t convert.

I have a feeling the listing is broken and not going to be fixed, but wanted to check if any of you wise folks had a suggestion before I pull the stock and create a new listing. I’d rather not lose the excellent reviews if I can help it.