Not receiving notifications on some of my followed threads?

[LeMansLambo14]

Hello, I have noticed something weird recently. Sometimes I do not receive a notification when a thread I follow is replied to, but it is inconsistent. As you can see from the screenshot below, a thread in the "Threads You Follow" section that is marked unread was replied to 5 minutes ago, but I did not receive a notification in the drop-down box. Not a serious issue, just curious because it is getting a little annoying. Thanks in advance!