You have enabled out-of-office delegation or queue sharing in Workspace, but the new tasks aren't forwarded to the target users' worklists.
Enable out-of-office delegation or queue sharing for the task in Workbench.
Open the AssignTask properties and select the "Add ACL for shared queue" option.
Open the AssignTask propreties and select the Allow Out Of office Designation option.
The out-of-office delegation and queue sharing functionalities only forwards new tasks that arrive after you enable the delegation in Workspace.