Is the new file for a newer hardware generation than the hardware you have? When Adtran has gone from Gen.1 to Gen. 2 in most cases the flash sizes have increased to accommodate newer and larger images.
Also, it may be necessary to erase the old image before writing the new one. If this is the case be careful not to interrupt the file transfer and writing process or you will have to go through a cumbersome recovery process. If the original boot image is erased and the box loses power or is rebooted, it won't come back on its own.