Retroflag NESPi 4 Case will boot SSD

Hi. I set up the SSD for HA with a USB3 adapter I had and plugged the adapter into the Pi and it booted fine. But when I used NESPi 4 Case SSD adapter it did not work. It is a Kingston A400 SSD. Rather than booting I just get the multicoloured boot loader screen (and no text).

Any Ideas?
Ben