iPhoto missing some photos on iPad 2

I have installed iPhoto app on my iPad 2. I note that it's is missing some photos in the library. In every album, the first several photos are missing and instead replaced by just white dotted-line boxes. Try syncing the photos with iTunes, but not working. How to restore these missing pictures?Then I get the "Updating Library" message. Now all albums are back to normal. The issue fixed.