At present, a summary task will display the complete pending approval status when the last to finish sub-task is complete - pending approval. In this state, you cannot add a task from the sub-task tab on the summary task. You can add a new sub-task from the project task tab.
This behavior is confusing for my users. You ought to be able to add a task in both areas while the summary task is complete pending approval.
1. You cannot add task from sub-task tab as shown in the context menu.
2. You *can* add a task from the project task menu as shown below.