strokes not appearing correct when previewed

hi can anyone help with this problem, hopefully pictures attached will make the problem clear. When drawing a strioke in Ai - it looks nice and smooth on the artwork view. When previewed the stroke looks "blocky". ie not following the same path as when in artwork. If i expand the stroke - it follows the preview shape (ie blocky, not the smooth curve i am looking for.

If anyone could help that would be great

TIA