The problem is that Amazon's TV apps (Android, LG etc) will show a movie is available in UHD/4K when it's available to buy/rent it in that format on their platform, when you click on "More purchase options" on the movie info page you'll see that the content you're watching is probably available in UHD.
Most of the time time it plays in HD for Prime Video subscribers.
It's pretty annoying and I hope they will show the resolutions separately in the future.
Luckily most Amazon productions are in 4K with HDR10+.
Did the title have the "UHD" tag?
UHD content have a separate page. HD and UHD is listed as 2 separate items
What’s your internet speed? Maybe it’s limiting your resolution to prevent buffer.
Low storage , try to delete stuff and reset the Amazon app Or other apps that use big chunks of space
Not all things come in ass 4k man they say it does but not always ... Now if every single thing says no uhd... Will say in description 4k
Paul Walker :(
Is possible that the movie you're trying to watch is not on 4K, almost only the Amazon Originals are in 4K Ultra HD.