A thumbnail is missing in the cloud

A thumbnail is usually missing because WordPress never created that size, or another plugin deleted it. Offloft flags it under Issues as "Image size missing", and the fix creates the size, uploads it and checks it. Your site keeps showing a working copy. A size missing only from the bucket shows as "Cloud copy missing".

Why new uploads don't run into this

Offloft waits for WordPress to finish making every size before it uploads. It waits for uploads to go quiet, 30 seconds by default, so other plugins can finish editing. You can change Quiet period in Settings → Uploads & sync.

Fixing many at once

Issues of the same kind can be fixed together. Open Issues and use the fix on the group, or run wp offloft fix from WP-CLI.