Newbie help, vellum to kindle

I use Vellum and am uploading epub and mobi to StoryOrigin. However, (some of?) my readers cannot download the kindle I have uploaded there.

I export from Vellum to epub/KIndle. Then use Calibre to convert from epub/Kindle to mobi.

Is there some plug in I should use to make this work? I can't find a pattern for my readers OS. Except they prefer to download for Kindle.