DAQ: Output of sample clock to PFI terminal does not work.

I have a National Instruments 6221 board and am using the session-based interface of the Data Acquisition Toolbox.

I would like to output the sample clock of a session on this board to one of the PFI terminals so that I can use this output to synchronize another device.

I tried the code examples provided here, verbatim, without success: http://www.mathworks.com/help/daq/clock-connections.html#btxsc18-1 section Export Scan Clock to External Device.

I get no errors and the data input works, but all the PFI terminals on the device stay "low" (I'm using an oscilloscope to test the output).

What could be the problem?

Thanks!