There are a couple of reasons for this. The most common is the boot-up sequence. When using MXI-3, the link cannot be established in time for the BIOS to recognize the cards unless the chassis is powered up first. (In the case of a multi-chassis system, make sure to power the chassis at the far end of the daisy chain first.)
There are a list of other possible reasons and solution on the MXI-3 Troubleshooting Guide at http://www.ni.com/support/pxi/mxi3.htm
Trey Hamilton
Applications Engineer
National Instruments
http://www.ni.com/ask
