ITM working on STM32F3Discovery but not on STM32F4Discovery

I have both an STM32F3DISCOVERY and an STM32F4Discovery.

wrote a basic program to just print to ITM.

As per the user manual:

soldered SB10 on the F3, ITM works fine.

soldered SB9 on the F4, ITM doesn't work.

Does the F4Discovery need something else done?

Slightly confused...