This is somethign that annoys me to no end, and is often the reason I don't demo my villages as often as I should.
After taking 60 villages in 3 days, and having to wait to start demo'n, the other 40 villages I had in my demo group got ignored, because it's so frustarting to click to the demo screen, then click the demo button, and have an error message pop up. I know it doesn't seem like much, but when you have 100 villages you need to build/demo, it can become a hassle. (That said, building on the overview and in the future demo'n there will solve most of that frustration).