Could be many issues. This thread lists many of them. SDlillini has these suggestions. Thanks!
Follow the chapter rules:
*Make sure no chapter markers are within two seconds of the start or end of the timeline.
*Make sure no chapter markers are within a transition.
*Make sure no chapter markers are within two seconds of the end of a transition.
*Make sure you do not have the “Share selected Clips” option checked when preparing to share to iDVD
Make sure you have lots of free space on your startup drive, 20GB recommended
Close iDVD, then:
Delete your iDVD plist (preference) file (Found in User / Library / Preferences) “com.apple.iDVD.plist”
Try save as disk image and burn that.