I've been cooking au gratin potatoes for 1hr 30 min and they're still not close to done, I have an oven therm. and the oven is @ 350, like the recipe called for, and there is still liquid in the dish... Does anyone have any idea what could be going on here? The recipe said they would be done in 1h30m. Should I add more milk and turn up the oven? I'm guessing they need another hour at this point.