AVI files not being seen by Plex server...

Until recently I used to use an IMac and it also used to run the Plex server at home. All the media was on an external USB drive that was connected to the Mac and every thing was working perfectly well.
For various reasons that I won't go into here, I replaced the mac with a Win10 machine, added the Plex server to that and attached the same USB drive to it that had all the media on it. Everything worked just fine until I tried to play some old .avi files that I've had for years (kids as babies, Muppets & Pingu shows that they still love etc etc). The material was just not there on Plex. I thought that they'd got deleted somehow and after a quick change of underpants, I realised that the files were actually still there but they were simply being ignored by the Windows Plex server.
How can this be? Why would the Mac Plex see and play the files and yet the PC Plex totally ignore them??
Any help would be greatly appreciated - granddaughter is a big fan of these shows and I need to get them working again asap!!