Help: Error Decrypting File

We encrypt over a 100 files named basically the same and every once in a while the vendor cannot decrypt a specific file. To test, we take the file and encrypt with our key and try to decrypt and we cannot. It says Error Decrypting File. The filename is XXXXXXXX_XXXXX_00002_XXX_20130605_150529 . X's are in place of actual name.

So we take the filename and remove the last underscore and the new filename is 2013060515029 at the end. We encrypt with our key and are able to decrypt. We then encrypt with the vendors key and send to them and they are able to decrypt. Why is this happening?

UNIX

6.5.1 version