Did you add any salt? Salt can toughen the skin of pulses if added at the start of cooking. Best to add toward the end of cooking or to cook the pulses separately and thn add to the dish. Hard water also lengthens the cooking times of pulses. Something to do with the calcium and magnesium present in hard water which is also present in the cell walls of many pulses, meaning it further protects the cell walls you are trying to break down. This is what happens with the salt as well! And it is possible that they are old, even if you have just bought them. If you carry on cooking, they should eventually break down. Liquidising will just result in a grainy dish if they're not cooked.