Check that mountvol.exe is located at %windir%\System32\mountvol.exe where %windir% is a system variable that resolves to the location of the window's system directory. It is usually, but not always, c:\windows (which would give a location of mountvol as c:\windows\System32\mountvol.exe). The library will run with whatever permissions and ownership you use when installing your software, so check that the mountvol command is available to that owner, and works with the owner's permissions. On a normal system, no special permissions are required.
Try rebooting the system, and retrying the command before loading the machine with other work. If a network file system is in use, quiescing the network is advised. If that doesn't work, perhaps a sys admin on the customer end can verify whether or not the system is functioning abnormally in some way.