Some photos are not syncing to iPhone 4

Solution 1:

Deleting the Photo Cache may do the trick.

To do so, just:

1. From the Finder, choose Home from the Go menu.
2. Once there, open the Pictures folder and locate the iPhoto Library file. (For Aperture, this is called Aperture Library.) If you have moved your iPhoto Library or Aperture Library to a different location, go to that location in Finder first.
3. Control-click the iPhoto Library or Aperture Library file and choose Show Package Contents from the shortcut menu.
4. In the next window, locate the iPod Photo Cache folder and drag it to the Trash. Important: Do not remove or modify any other file within the library package. Removing or modifying other files may cause data loss. Close the Finder window.

Extracted from iTunes: Understanding the iPod Photo Cache folder.