Galleries I deliver have some images showing up as grey boxes only. When I attempt to make a minor edit on the image, it will then re-sync and show correctly, but another will disappear into grey. This is extremely inconvenient for my schedule and business.
When the grey thumbnails appear does the cloud icon show that Lightroom is still syncing or are you seeing that it is fully synced with a green checkmark?
Interestingly enough, I went to share the links that a few of my clients were complaining about the grey images, and they all seem to be there fine now. This was a very odd 2 days when I posted the original post on this board because I spent a lot of time trying to see what was going on. I created PDF pages for them to review. I've since then decided to export my images as JPGs (with watermarks) and re-import them as JPGs which I never had a problem with. I will gladly share a new gallery if it happens again.