Why aren't my potatoes cooked? I had a 3lb roast on a bed of baby potatoes (smaller than a ping-pong ball but larger than a grape) and onions at 375F for almost 3 hours. The roast came out great (cooked to 150F internally), but the potatoes are still raw in the middle. Shouldn't they have cooked by then? And what can I do in the future to prevent this?

As per comments: There was also a pizza stone in the oven. Could that have played a factor? It was preheated with the oven, as suggested in Storing Pizza Stone in Oven. The oven was preheated about an hour (my oven doesn't beep, just has a change in light color, so sometimes we forget we were preheating and wander off).
Put the roast on the bottom and the potatoes around the edges. Cover everything with red wine (or whatever fluid you are using)