You were using the wrong recipe or didn't let it cook long enough! This recipe used tomato paste to help thicken the stew and the long cooking time gives the stew time to evaporate some of the water as well as the starch in the potatoes to break down and thicken the stew. In the notes of the recipe you can also read about how to add cornstarch to your stew if you want it even thicker! No running beef stew here.