I’ve found you can extend the life of the plunger by placing the plunger rubber-side down on a table or counter top, tilting it slightly onto the edge of the rubber and then rotating it. This essentially presses the rubber and extends it outwards and seems to help achieve the seal you expect. I’ve been doing it for a while now and it’s worked well for me.

I’m sure it won’t last forever but it’s something worth trying to get a bit more life out of it.