Can't convert certain books to MOBI

Hi,

First post here, please be gentle. :)

I've been using calibre for years without issue, until today. I have a handful of Kindle books that I just can't seem to get the DRM off (around 11 books). I'm trying to reorganise my library. I'd got lazy in the last few years and had some books DRM-free and a few I had sent straight from Amazon. But now I want to organise things and have all my books DRM-free. I selected all my books on the device and moved them into the library (which I'd emptied before this) and then highlighted them all (around 170 books) and converted as bulk.

The 11 books that won't convert had error messages pop up to tell me so. At first, I thought maybe converting in bulk was the issue. So after the rest had finished, I selected each book on its own and tried to convert, but still the same error message about DRM.

I am using the latest version of plug-in tools from ApprenticeAlf's website, and successfully converted a book I bought from amazon just two days ago.

Can anyone please help me? The format for the books that won't convert are in AZW format and AZW3.

Thanks.