Problem checking out objects

Hi all, I'm new to ER BA. My team is modeling business processes and would like to split the work up. However we cannot check out individual models or diagrams within the workspace. When we select Repository -> Check Out Object(s), nothing happens. The object is not checked out. In order to work on the diagram, we need to check out the entire workspace and that means only one person can be working at a time. I tried disconnecting from the repository in an attempt to work locally but the entire workspace still shows checked in and locked. Does anyone have any thoughts or suggestions?