Welcome, Heather.
I suggest you install
WinFF. It's just a GUI for FFMPEG, but it might help in two ways...
1. WinFF includes a current version of FFMPEG. Updating the one used by PVD might help. Copy
ffmpeg.exe from the installation directory of WinFF to the FFMPEG subdirectory in your PVD installation.
2. If the screen shot maker still doesn't work, WinFF might shed some light on the problem. Try playing the file using WinFF. If FFMPEG can't play the file, then it's unlikely it can make screenshots.
The purpose of WinFF is to convert video formats using FFMPEG. That's not directly relevant to your issue, but you might find it useful if you're handling a bunch of different formats. If WinFF/FFMPEG doesn't work, I suppose the next thing to consider is whether the file can be converted to another format. Use
MediaInfo to determine what format a file is in.
Here is a useful resource for learning how to do various conversions.