Cirrus,
Could u attach the VI that you are using?? ( probably you have not selected the Mode as 'RSE' while configuring your AI channel)
Did u measure the voltage on same channel using Measurement and Automation explorer?? if yes, what did you observe??
And a little more information about the type of signal source ( wheather it is a floating/ground referenced signal) and its voltage range would be helpful in answering your query.
Regards,
Dev