Milk can froth if the milk is fresh and has a lot of fat content. If milk does not have enough fat, it will bubble instead of frothing when steamed or heated.  Fresh milk contains more active enzymes that are needed to encourage the formation of foam. However, once milk is past its expiration date or becomes sour, it cannot foam properly. In addition, milk that has been pasteurized will also foam less than unpasteurized milk. This is because the heating process reduces the fat content and kills enzymes responsible for foaming milk.  However, this does not mean that your milk can't still be used for making froth or steamed beverages. You can purchase a milk frother or steamer to help you with this process. Another reason your milk might not be foaming as expected is if it has been overheated while steaming. If the liquid becomes too hot, the proteins and fats will form clumps instead of creating foam. This means that your milk might bubble instead of frothing.