I'd strongly recommend using a food thermometer like this one to ensure the water or milk is around 40'C - skin temperature. Too hot and it will kill the yeast, which means your bake will not rise. When left in a warm spot for 5-10 minutes, the yeast should form a frothy top a bit like the head on beer. If it doesn't your yeast is most likely out-of-date or has been open for too long. Discard it and start with a fresh pack.