Default "Error Flashing Boot Image" When restoring Rom Manager Backup.

Ok, My phone had some issues with powering down so i got it replaced, i have the new phone, it runs fine, but for some reason, whenever flashing a nandroid backup, it fails at restoring boot image. Then the phone boots straight into the boot loader. I installed a stock update.zip with root and am running fine now but why can i not use my nandroid backups? cant i use them on other phones?