Jess:
Two things to do here:
1) This sounds like a bug, so you should probably report it to the CytoCluster creators. The app page at
http://apps.cytoscape.org/apps/cytocluster lists the maintainers' email addresses under the Contact Information section.
2) You could try creating a new network from the selected nodes and edges (
File->New->Network->From selected nodes, selected edges), and run CytoCluster on that network.
Bill