I was finally able to figured it out. I had the "watch" option checked. Apparently it's not possible to both "watch" and "notify" a single thread. At least, not for me. Now that I've "unwatched" the thread, I'm able to "notify" it instead. I'm not sure what "watching" does, but it was not generating emails for me.
Are you sure about this? I can do both watch and notify the same thread.
Watch adds the thread to the Watchlist so you access them with a single click.
Notify sends emails to you whenever anyone posts a reply on that thread.
You're right. It's not that simple. If I "watch" first, then I can't "notify", but if I watch and notify in the same action, or if I "notify" first then "watch", it works. Strange...
By default when I created my account I was automatically "watching" all threads I posted to, which blocked me from being able to "notify" on those threads. But I could "notify" on other threads. So by unwatching and rewatching the two threads I'm now able to watch and notify on all threads.