Thank you for using Apple Support Communities. It seems like you are able to stream videos in HD but if you try to download the video it is not in HD. I would recommend you read this article and use it to help troubleshoot the issue.
Download content in 1080p on iPhone, iPad, or iPod touch
- Open the iTunes Store app.
- If you're not signed in, sign in with your Apple ID.
- Search for the movie or TV show that you want to buy, then tap it.
- Tap the Buy button to buy and download the item.
Download content in 1080p on Mac or PC
- Open iTunes.
- Click the Store tab at the top of the iTunes window.
- Search for the movie or TV show that you want to buy, then click it.
- Click the Buy button to buy and download the item.
Depending on your computer, you might not be able to buy, rent, or play content in 1080p. If you see a message that reads "This computer may not be able to play 1080p videos. You can still download these videos for use on other devices," 1080p content won't play on your computer.