ICX7450 with ICX7400-4X10GF 4-port 40G Module > Clients are only connecting at 5G, not 10G?

[benjamin_whiteh]

We have an ICX7750 with 48 port 10G module. All clients connect at 10G no problem.

The same clients - Mac High Sierra & Windows 10 - when connected to our ICX7450 with ICX7400 module, only achieve 5G speeds. 

The module is recognised ok by the switch, and the ports are running at 10G Full. Auto neg is not available on these ports - I think they only run at 10G.