why can't I download a video from a shared album

Can you view the video in the album or play it?

There are several possible reasons for being unable to download a video:

• lack of storage on your device,
• or an incompatible video format,
• or the owner may have removed the video from the shared album
• or there may be a problem with your internet connection or your Apple ID
• or there has been a temporary problem at Apple's cloud servers. You can check the status of the APlle services here: Apple - Support - System Status