Notification alerts (on desktop) missing after a day or so!

I went to go reply to a comment someone made to one of my comments on a video.

I went to the video to see if I could find my comment, but mine doesn't appear in the "top" or the "newest" since I commented on April 4th.

So I thought I'd just go to my notifications and find it that way, but the notification is nowhere to be found. Notifications ranging from yesterday to 3 weeks ago are there, but not the one that appeared 2 days ago.

Can someone explain what's going on?