Ah okay. I mean, I also haven't had to open a carton of milk since I was in school, so it's entirely possible if I tried today I would have the same trouble. Like maybe companies nowadays use different glues or cardboards that behave differently.
Cereal boxes are like that, for me. I used to be able to open the flap with no problems but lately it feels like I can't do it without ripping the box flap in half, so that it no longer closes nicely with the little tab-slot and I usually have to cinch or roll down the inner bag or even put the cereal in some other container to keep it fresh.