If there is frost on just 2 or 3 rows then it is low on gas. There should be frost from bottom to top. Today's models that use R134a tend to get a tad low. They only hold a small amount to begin with. More than 1/3rd of a 12 ounce can would choke down the compressor. If you have no guages a good way to guess if you have enough is to slowly add in some freon till all the coils are frosted. If the compressor ear frosts over you have too much in there. Add the freon slowly.