Crock pot help- why is my roast dry?!?

The last few times I have made pot roast in my slow cooker it's been terribly dry. I am getting about a 2 lb chuck roast everytime, cooking on low for about 6 hours. adding potatoes/onions/carrots/seasonings and 1/2 cup water. the recipe tastes very good i've made it before, it's just the meat's lack of tenderness that's making it harder to enjoy. any ideas? am I cooking it too long or is my roast too small?