Problem exporting photos

Hi, can anyone help me out with a problem that's only just arisen with Lightroom on iPad? When I try to export a photo back to camera roll or anywhere else I get a box popping up saying it's downloading smart preview and it may take a moment but it just keeps on a loop trying to download smart preview, so I can't get any photos back out of Lightroom. It used to work fine. I looked for an update to the app but it seems to be up to date. Thanks a lot. Ian