Hi Michael,
From what I understand you are saying that GATT_Notification is not returning the SUCCESS status? Or maybe it's indeed returning SUCCESS but you can't send data because the UART is blocked somehow? If you are calling the UART to write in it's own callback you should be falling in an infinite loop or something that hangs your device.
Confirm if you can send 2 or 3 notifications in the same connection using your cases. Normally I would say you to go into debug mode but you can't since you're using the debug header port for UART. Hope you can solve your problem.
Best Regards.