If the tubes are too small, they'll stretch too much when you inflate the tire, and be vulnerable to flats. Even though you have 26" tubes and 26" tires, tubes are meant to accommodate a certain width range. Check the label on the tire, check the size range on the tube (it should be printed on the box), and make sure your tubes are meant for tires as wide as your Nokians (26 x 2.1?).