after you wrap them in moist, not wet, paper towel, did you put them in ziplock bag? otherwise, the tip will dry out. the white spots you are are usually where the roots might come out. you want the whole cuttings to get constant humidity, but not wet condition. meaning, put them in ziplock back wrapped with moist paper towel and air it out once a day or so. if you don't air them out, they will mold.