I mean there are movies that can be downloaded and there are movies that can't be downloaded
Qlive
Well, most thought your issue was a subtitle issue, including me.
If it's specific movies, not downloading, then we need the titles of those not downloading.
Only list movies or items that do not require renting or buying. Only ones included with Prime Membership.
Then they can be tested by others, or if/when I have time, I can test any movie/show that's included in Amazon Prime membership.
If it only happens with rent or buy movies/shows then first uninstall and then install streamfab, then delete the streamfab.log
Then try trouble item(s), you could then try attaching the log here, but you may need streamfab support to help.