Over time, the feed rollers wear smooth and lose their grip.
Try rubbing the paper feeder rollers (look down into the printer where the paper path is) with a pencil eraser. The erasers on the top of pencils are ideal as the pencil provides a handle.
If that doesn't work you could try cleaning with an alcohol soaked cloth. (for alcohol read an alcoholic spirit like vodka or use isopropyl alcohol (IPA) obtainable at good pharmacies or from Maplin)