Playing downloaded movies on my Mac

I have a lot of downloaded movies that I purchased from Apple. I used to be able to watch them on my Mac, but I suspect that there's been some encoding changes because now I get the dreaded error message that the movies aren't playable on my monitor. I have no idea why my monitor (an LG UHD that I've had for years) should have anything to do with it. I purchased the movies and now I can't watch them unless I stream them. So in effect, I purchased nothing; I'm leasing the movie for viewing online. Movies not purchased from Apple play just fine. I assume there's no fix for this. It's just upsetting that I payed for something that I apparently don't own.