you will have to capture a sniff in front of the css to see if it is receiving traffic for the vip.
You can also do a 'sho summary' and see if the hit counter is increasing.
Then check the upstream device and verify the mac address associated with the vip.
Make sure the L2 cam table is pointing to the CSS.
Do you have redundancy turned on ?
Finally, not sure what is the colaboration suite exactly, but if this is a software loadbalancing tool, you may want to stop using it if you have a CSS. Simply configure the server independent from each other.
In the past we have seen many issues when trying to combine both software and hardware loadbalancing.
Gilles.