In this exchange, I had occasion to comment in answer to a specific person (in this case, "Alex B"). Normally, when I type '@', there's a popup to suggest a completion, but in this case there was not, and even when I typed in "@alexb" manually, that disappeared when I submitted my comment. Am I doing something wrong? This has happened before, with no pattern that I can recognize.
You did nothing wrong, and Alex B. was notified of your comment.
When you leave a comment under a post (a question or an answer), the original poster (OP) of that post will be notified. No one else in the same comment thread is notified, unless you add an @-ping. That is, your comment will give a notification to the OP and at most one other person whom you choose to ping.
If no one besides you and the OP have commented, it is clear that the comment is aimed towards the OP. Therefore in such cases the system automatically removes the @-ping, and it also doesn't suggest it when you type "@a" for a name starting with "a". (I may or may not remember the exact criteria for @-removal correctly, but this is the gist of it.)
Although the @-ping is not required when you could only be addressing the OP, I think it's confusing that it is removed and not suggested. I would give more value to consistency than efficiency. I have seen this point discussed at the mother meta, and that is the place where one should go if one wants to change a feature like this.
For details, here is the mother meta FAQ post on this topic. Thanks for finding it, Andrew!