You will first need to break the system down into it components to see where the failure is occurring. The first thing to check is whether your DAQ card is returning the appropriate data. You can check this by opening up Measurement & Automation Explorer and testing the hardware there.
If the hardware is working okay, then you will need to dig into the software to find the problem, and to do that you will need to open the VI in the LabVIEW development environment. First make sure that you are using proper error reporting in order to catch any errors that may give you clues to the breakdown. After that, you will need to use probes, breakpoints, and execution stepping to pinpoint exactly where in the program things start going awry. Once you've narrowed it down to a specific place, it will be much easier to troubleshoot. Fill us in with what you find out!
Regards,
E. Sulzer
Applications Engineer
National Instruments