I was noticing the same thing, and what I discovered was that Primary was backing up and shipping to secondary just fine, and restores were being attempted. However, since there were zero changes, everything was being skipped. I forced a change on Primary, and it found the correct file and restored. No error, and alerts went away.
The bummer of all this is that it's not smart enough to log the attempt. So if you have a small database with zero changes say for 2 hours, and you set your alert for 45 minutes, it's going to scream at you often even though there is no issue.