It's possible your iPhoto Library has problems beyond the Rebuild function's ability to repair. An alternative could be to back up
* the Library and rebuild it with
iPhoto Library Manager**. iLM's approach actually recreates the Library using iPhoto's albumdata.xml file, and may yield a properly working Library, albeit without items like slideshows, books or calendars, but with albums and keywords, faces etc.
* iLM doesn't overwrite iPhoto's original Library, but any operation with the latter warrants a backup.
**) Fat Cat's iLM page mentions a trial download, but doesn't specify limitations.