Crock Pot Cooking: Why are my roasts chewy when done?

[LashCatt]

I have recently just started crock pot cooking, so far I have made two roasts, round roasts in my crock pot and both times the meat seemed chewy when done. They were both a little less than two pound roast and I cooked them both with assorted veggies on high for a couple hours. I can figure out what I am doing wrong, any one have any suggestion for making a crock pot roast that isn't chewy when done. Thanks in advance.