I found what my problem was. The 'Convert CVI fp file' function was creating LabVIEW VIs that had errors in them.The enum values that are passed to the DLLs occasionally require a look-up table in the VI, and this table (integer array) is defaulted to empty, so invalid values are ALWAYS passed to the DLL. This is probably a problem with LabVIEW 5.1 or the version of the driver that I had.
Thanks for your help, anyway.