I was once given a batch (in a GW blister) of greenstuff for my birthday, almost 15 years ago. I'm not the most prolific sculptor, so I barely used it (also because I prefer ProCreate myself).
But when I did use it, I found it increasingly difficult to work with, which caused me to use it even less, etc.
Then I needed to fill an order to above a certain amount to get free shipping, so I decided to also get another batch of greenstuff.
And what a revelation that was! It was so soft and pliable! It actually made sculpting fun again

I also have a huge roll of ProCreate which I incidentally used just this weekend. I had ordered it last year and forgot I had it until recently. The two components are stored in a plastic tube, with the two parts individually wrapped in plastic, and both were hardened towards both ends.
So yeah; greenstuff, but also ProCreate and most likely all other two part sculpting materials, tend to dry out over time and get harder, even when not stored together, in the dark, at cool temperatures and in an airtight container...