Logs, a few sample movie links available to all who have the Amazon Prime subscription may get you further…
Last I checked, Amazon, when available, downloads SRT; you can download it as a separate SRT file or remux it into the video file.
It could even be just those certain videos not working. or not having SRT, but as I said, most of my Amazon comes with SRT; I switch between remuxing into files and saving them separately.
Maybe even go back and forth between MKV (ffmpeg) and MKV (mktoolnx) or even MP4 (ffmpeg).
Try also clearing temp folder, clearing history, etc.
Check here (link below), and the more info you can provide will make it easier for someone to help.
https://forum.dvdfab.cn/d/433509-how-to-post-issues-in-the-streamfab-section