Okay, I'm happy to explain furter.
In Settings -> Discussion, you can set a Maximum Rating which is what that particular blog will allow in terms of commenter Gravatars.
The Maximum Rating setting is configured for each blog.
If you changed your Gravatar rating somehow from G to PG, and a blog where you left comments had a Maximum Rating of G, then the following would occur:
- your Gravatar would not appear
- the blog would display a default Gravatar
If you aren't seeing your Gravatar in a comment, can you reply with the post URL so I can take a closer look?
