windows 8 - How can I force the Pictures app to refresh its library?

I've imported some pictures to my "My Pictures" folder using Adobe Lightroom, but the Pictures app doesn't seem to notice the new files? Is there some way to force it to refresh its library?

To be clear,

Pictures are sorted under C:\Users\\Pictures\ by year, then date; for example: C:\Users\\Pictures\2012\2012-10-20, these show, however those in C:\Users\\Pictures\2012\2012-10-29 do not, despite coming from the same camera, being in the same format, thumbnails working in Exporer, the pictures opening in the Pictures app via a double click in explorer, and Windows Media Centre having no problems.

I have tried excluding the Pictures library, and reincluding it, but that had no effect. It has taken three or four reboots for even the folders to show up in the app, but it reports them as empty

Answer

This can happen if Windows Search hasn't had a chance to index the photos yet, or the index is corrupt. Rebuilding the index can force the pictures app to rebuild its own index.

to rebuild the index, or ensure the relevant folders are included from the Start screen, type Indexing Options and choose the option of that name that appears under the "settings" section.

If the option doesn't appear, it may be due to the search service having been disabled (for example, to install Outlook 2013 on x64), so it would need to be reinstalled from control panel.