Why is my yeast so slow? Yesterday I made a batch of dough for a bread-like treat. It was a mix of cooled-off mashed potatoes, flour, instant yeast, room-temperature milk and room-temperature water.

According to the recipe it should rise in a warm place for about 30 minutes. So I placed it near a window with a hot towel on top for about an hour in the end, but nothing at all happened. I tried baking some of the resulting dough anyway, but it was not working at all.

So after that I gave up, turned down the central heating and went to bed a bit disheartened. I left the dough on the counter, in an open bowl. When I came back this morning, the dough had risen to at least twice it's original size, maybe more. 

How come the dough rose to such a size overnight and what should I change to make it happen in minutes instead of hours? Am I underestimating what a "warm" environment is? (The house is usually at around 20C and the radiators are usually off, is that too low?)

And if the dough rises slowly and openly like this, is it still okay? I'm planning to try and bake it again when I get home from work, but it'll have been in the open for close to 24 hours then.
There are so many factors in the rising of bread, have a look at this previous answer for something more exhaustive. I will say that 30 minutes is pretty short, especially when you are starting from a cool dough. You are using milk, which will enrich the dough, and although you haven't mentioned it I'm sure there is salt in the mix, both of these will slow yeast development. 

There's nothing wrong with this, in fact slower is better for good gluten and flavor development. I would be prepared to let an enriched potato dough prove at least 3-4 hours, or overnight in the refrigerator. 

24 hours may be too much at it may have over-proofed, also foodborne illnesses may have developed in that time. Personally I'd discard it and try again.