If you are using the DOS version of InSight, try shutting Windows down and restarting the computer in DOS mode. Run InSight from the DOS prompt and try again.  This will typically only work on Windows 95 and 98 systems.  If running Windows 2000 or XP you must upgrade to the latest version for compatibility.

Also check the following: Make sure you are using a genuine Sigma PN1727 cable which has yellow shrink tubing on the meter end. Make sure the baud rate set in the sampler or flow meter matches the baud rate set in InSight. Check the COM port to be sure the physical connection matches what is shown in InSight. Or try one of the other COM ports by changing the choice in InSight; if COM 1 doesn't connect, try COM 2, and etc.