Are you still signed-in? Only then the sync check box will appear.
When I look at the back-end of your account it looks like that there are still pending assets needs to be sync for the collection "PAGB Collection". What status is Lr Desktop showing when you log-in to your account
Hi Guido, thanks for getting in touch. When I fire up Lightroom under W7 and check Preferences and Lightroom Mobile I see my Adobe ID with the "Sign Out" button next to it. If I check out the "More Account Info Online" I get taken to my account page. If I hit the "Adobe" option from the account screen and the "Lr" option from there I get to see the same 12 images that have appeared on my iPad, but there are actually 34 images in that collection, presumably the remainder being those 'pending assets' you mention. I am not presented with the sync check boxes I would expect to see in the left hand column of the Collections area.
So I've now signed out of Lr Mobile from W7 desktop, restarted Lr and the logged back in to Lr Mobile. Lr Desktop now seems to be picking up my iPad Camera Roll, which is something I hadn't asked it to do, although it seems to have stopped at 45 of 46. Over on "Lightroom.adobe.com" the number of images in the Camera Roll file is increasing but not caught up with the number on the PC yet. There are now two PAGB Collection files, one with the same 12 images, the other is empty. Over on the iPad I'm also seeing two PAGB Collection files as well, same content as on "Lightroom.adobe.com".