Content of Shared Album Photos Disappear?

The content (pictures) of my shared albums in Photos app (High Sierra) disappear after a short while of app being open. I then have to Quit and restart app to see the contents appear again. Any thoughts on how to remedy this? I'm grateful they don't permanently disappear, BUT having to quit and restart every time I want to see them is a bit of a hassle. Thanks in advance for any advice!