Kirby Sentria doesn't pick up visible dirt

Question

My Sentria obviously has suction - if I tilt the head up, the carpet comes with it. However, it doesn't pick up even small tufts of dog hair, small bits of yarn, bits of vegetation. The carpet underneath looks cleaner - but there's still stuff on it. Surely I shouldn't have to pick that up by hand?I am using it on a low-pile area rug with head in position 1 or 2.Also, how do you remove the bag? I know to rotate the bag at the top, but gentle or otherwise, the cardboard bends but the bag doesn't budge,