Jetpack was working fine for me, until I must have accidentally de-activated it (though I don't remember doing this).
I activated it again, connected to WordPress.com again - but Jetpack seems to think it was never active before, starting my stats afresh.
I have previously been using WP.com Stats until it stopped working a few weeks ago. So the question is... how can I bring back Jetpack and make it remember that it was already actively collecting stats?