Hi,
As Joerg says, make sure that your PluginFactory implementation is able to find any object of your plug-in by its type and its id (any object that you want to pass between workflow elements or that you want to hold when a workflow is suspended), otherwise it won't work.
And just one thing to add to Joerg's comment. Since version 5.1 of the vCO Plug-in SDK, the vCO Plug-in Developer's Guide is also available online as a PDF here: http://communities.vmware.com/docs/DOC-20394
I hope it helps.
Sergio