I'm guessing "Somebody agreed" applies to the forums and "commented on a comment" is for articles?
The "someone" that had comnmented but got blanked out, was Pikashoe. For whatever reason, if the person replying was the author of the first comment in the thread, it wasn't creating the required data needed for the notifications.
This has been fixed - unfortunately, it's not retroactive so those blank ones will remain blank. If you see any new ones though, please let me know ASAP...