You're not stupid, they are a lot harder to get apart than I expected. Give them a really hard tug.
I guess they have to be that stiff to stop them sliding when in use.
They are really uselful, although I wish I had bought two lots for some of the sites I've stayed on.
We nearly bought some of these a while back but thought they looked a bit precarious if you used more than about 2. Plus we had visions of spending hours trying to get them apart every time! Back to the blocks of wood!!