My alerts are broken and aren't always sending like they are supposed to.
When you receive an alert from a watched thread, the system prevents any future alerts from that thread until it is flagged as read — which happens when you read the LAST post of the thread. Not always the post you got the alert for, and never the alert itself. That's why the alert points out that "There may be more posts after this."
The quickest solution is to read the last post of the thread, and your alerts for that thread will be fixed.
If you think that you may have missed something, check Watched Forum and Watched Threads, which may show where there are replies you haven't read.