This occurs because of a conflict with Task Status. When defining the task swim lanes the starting Task status value is set to “Ready to Start”.
The new Task does not appear in StarTeam Agile but it does appear in the StarTeam Cross Platform Client
CPC (Note that the status is Pending):
Setting default Task Status value back to ‘Pending’
If a Task Status value is not set when entering the Task, StarTeam Agile defaults the Task status value to ‘Pending’. As the starting status has been set to "Ready to Start" this effectively hides all tasks with no Task Status value set.
This have been resolved and a fix will be implemented in StarTeam Agile HF1. Contact the StarTeam Support team for availability.