I've wondered this too. I'm 54 and just started Oestrogel at long last after an exhausting battle with my GP.
I'm 2-3 years post meno now and two pumps was only enough for two weeks, then my body got used to it I suppose, symptoms came back, I'm on three pumps now.
It's the progesterone that is difficult for me, you don't mention it, do you take one?
If so, that is a more likely culprit imo.
Sorry I can't answer your question, I just hate to see no replies to a post!