There is a bug in DataServices where it will randomly truncate the path names as defined in your substitution variables. (UNC paths)
I cannot say for sure that this is your issue, but myself and my admin banged our heads against this for a couple hours one day. He watched the file request on the server and the path was clearly getting chopped off. The really weird thing was that if you changed the path to something shorter, it STILL truncated it. Then if we changed the base folder name to something elseā¦ ya then it worked just fine.