I take it you are using an 8.9 system? Have you installed SP6? With the later service packs of 8.9 you have to define which network card on the logger you are using for your spanning, if you have not done this then it wont work.
Additionally you need to configure the switch to correctly span/mirror the data to the required network card. There is a whole chapter in the 8.9 VoIP logger installation manual dedicated on how to configure span ports correctly for several cisco and avaya switches.
Although an installation engineer I've never actually performed the switch configuration myself, I just tell the switch engineers what they need to do and on most occasions they do it correctly, every now and then they have issues and these seem to be due to the way the vlan is configured or something like that. Sorry I can't be of much more assistance.