most probably just not enough green vegetables. don't worry oily is good! much better than dry. also the more active you are the more oil gets released through the skin. young people having faster metabolisms and being more active have oilier skin than oldies. fasting slows down the oil coming to the surface cos its using it for fuel.
when i first started juicing green vegetables my system wasn't used to such concentrated doses of alkalinity and my skin went temporarily dry while it adjusted. so dry in fact that my fingertips couldn't pick up smooth things for about 3 months. totally weird. talk about hands on evidence of how alkalinity changes the oil glands :-). once the body adjusted the skin became moist and again, but never quite as oily.