why is my Iphone not backing up

This may be caused by a corrupt existing backup that needs to be deleted, or by data on your device that is causing the backup to fail. To troubleshoot these, try deleting your last iCloud backup (if you have one) by turning off iCloud Backup in Settings>iCloud>Storage & Backup, then tap Manage Storage, tap your device under Backups, then tap Delete Backup. Then go back and turn iCloud Backup back on and try backing up again.

If it still won't back up, you may have an app or something in your camera roll that is causing the backup to fail. To locate which one, go to Settings>iCloud>Storage & Backup>Manage Storage, tap the name of your device under Backups, under Backup Options tap Show All Apps, then turn them all to Off (including camera roll) and try backing up again. If the backup is successful, then the camera roll and/or one of your apps is causing the backup to fail and you'll have to located by process of elimination. Turn the camera roll On and try backing up again. If it succeeds, turn some of your apps to On and try backing up again. If it succeeds again, turn some more apps to On then try again; repeat this process until it fails. Eventually you'll be able to locate the problem app and exclude it from your backup.

Yes. Suggest you import your camera roll photos to your computer and delete them from your phone, as explained here: http://support.apple.com/kb/HT4083. You can then put the photos in a folder and sync them back to your phone by selecting it on the Photos tab of your iTunes sync settings and syncing. They will then be on your phone, but in a folder that is not included in the backup (photos sync to your phone from your computer are not backed up because they are already backed up on your computer). Then try leaving the camera roll on in the iCloud backup and hopefully the backup won't fail again. (This assumes there were photos in your original camera roll that would not back up that have now been removed from the backup.)

Just be aware that to remove photos synced to your phone from your computer in the future, they will have to be synced off your phone using iTunes. They cannot be deleted from the phone itself.