For multi-session Customer Journey, a matching groupId must be set for each associated Context object. You can see this groupId value in the Contexts metadata.
ReST request to check Context metadata (note, the full ContextStoreRest API is available in the Context Store Developer Guide) http://CLUSTER IP/services/ContextStoreRest/cs/contexts/meta/WorkRequestId
Verify that you can retrieve the Journey data through the ContextStoreQuery interface itself, leaving Visualization UI out of the equation first to simplify troubleshooting.
ReST request to ContextStoreQuery to retrieve Journey information http://CLUSTER IP/services/ContextStoreQuery/edm/contexts/audit/groups/context/WorkRequestId