carbonation of cider kit in keg

hi i am new to this. had my first batch of john bull cider, first formenting stage went ok, second fermenting, i bottled six bottles of it with a tea spoon of sugar in each bottle, the rest of it i transferred to a 40 pint keg. The bottles after a week and half seemed to have some sparkle to them but the keg after 3 weeks was completely flat cider and it should of been sparkling cider. Any help please as i am still learning.