If the SheevaPlug already boots, then one must assume your image copy card is not imaged correctly, if it boots from the original fine.
Since you've correctly imaged from the card Image I've provided, then I have to assume you've copied your working card wrongly, or the image writing to the new card went wrong.
Perhaps Try and zero out the card you are imaging to prior. Same brand/make? Provide a serial console dump of the boot process?
I have insufficient data otherwise to assist further.
Need much more details and data