Hello sheepqueen,
For difficulties with the video for your purchased movies, you'll want to take the steps below:
Update to the latest version of the Apple TV app
Update to the latest version of iOS or iPadOS, macOS, or tvOS.
If you're using a compatible smart TV, streaming device, or gaming console that's connected to the internet, the Apple TV app should update automatically. If you need help, you can contact the manufacturer or learn how to get the Apple TV app on your device.
If the video quality is bad or it stops playing
1. If multiple people on your network are using the internet, your connection speed might slow down. Ask them to temporarily pause their internet activities.
2. If you're using a smart TV or streaming device, turn it off and back on again. If you're using an Apple device, restart it.
3. Tap or click the download button to try downloading the movie or TV show instead.*
* You can't download movies or TV shows to Apple TV, smart TVs, or streaming devices. You can download HDR content only to certain iPhone, iPad, and iPod touch models. You can't download 4K video content to any device.
You can also find those steps in this resource: If you can't watch or stream movies and TV shows in the Apple TV app - Apple Support
Should that behavior still persist, you'll want to reach out to Apple Support so they can look into this further: Contact - Official Apple Support
Take care.