Help! Switched to Mac and Now Half my media will no longer play!

Hi, I recently switched from Windows to Mac. Got everything set up. Transferred my library onto a new hard drive. Let everything re download and update and got ready to watch some movies. But unfortunately what I discovered was that about 3/4 of my movies and a few of my TV shows will no longer play. Plex says it can’t find the files or server disconnected and a whole host of other error messages. Nothing of which is helpful. Sometimes the movies will play on my new iMac but not my Roku hooked up to my TV. And sometimes it is vice versa. Sometimes a movie will play on the Roku but not my iMac. The media player I downloaded on my iMac is terribly buggy, but it works fine on my MacBook Pro. I really like Plex and want to keep using it, but I cannot deal with bugs. I have checked the files that won’t play and they will play fine in other movie apps like super player lite and quicktime, so they are not corrupt. I tried to delete the libraries and rebuilt them and Plex would not allow me to do that. Nothing happened. The circle spun for a while and then went back to normal. I tried re downloading a file and replacing the old one and it still would not play. I tried converting the file to a different format… but you guessed it. Still won’t play it. If someone could help me figure out what the issue is, I would sure appreciate it!!!