Not all videos are uploaded in 4k resolution – it's creator's preference. If the video was recently uploaded in higher resolution – like 4K or 1080p, this usually takes longer to process. Let us know if that helps, or tweet back @ us if you have additional questions!
Thanks for clarifying, can you try refreshing, closing & reopening the app? You can also try uninstalling & reinstalling the app. If the problem persists, we'd recommend sending a feedback report directly from YouTube: https://yt.be/help/feedback . Hope this helps.