There is a single message that I can see online on my Gmail account, but won’t download to eM Client. I have previously had not issues (that I am aware of), downloading messages in general, and this sender in particular.
Thoughts please
That can happen when the message header is still in eM Client, but the message has been deleted or moved on the server.
You can right-click on the All Mail folder and choose Properties > Repair. This will resync your message cache.