Generally, the name should auto-complete for people who both:
- Can be notified. These are the users who commented on the same post.
- Are not notified by default. The author of the post is notified by default.
Also, you can have only one explicit notification in your comment. So if a name does not auto-complete, this normally means that either:
- They are the author of the post, and get notified by default.
- They cannot be notified because they did not comment before you.
- You are trying to notify more than one user.
If it does not behave that way for you, you might have encountered a real problem.
There is an interesting exception to the above. Users who edited the post you're commenting on can be notified, are not notified by default, but their name does not auto-complete.
There are some other subtle aspects, e.g. in the case where the author of a post responds to a comment on his/her own post. For the full glorious details, see the answer to this question:
How do comment @replies work?