as long as the save data is from the same account it should be ok but if you changed to a different psn account or even user name it might be why .
when you signed up for playstation network on the new ps3 in order to use saves you should have selected to use an existing account and used the same user name and e-mail/password as on the first ps3 then copy the save to your ps3 to use it . it can't be used directly from a usb stick