Try taking a look at the DB-9 connector to make sure the pins look okay on the units that you are having trouble with. Beyond that, with several working FP-1000's and one or two that do not work under with the same DIP switch settings and cables, I would recommend that you contact National Instruments for a RMA number for the units that can not be found.
Regards,
Aaron
LabVIEW Champion, CLA, CPI