ESXi 5.5 image incompatible

After upgrading vCentre and Update Manager to 5.5, importing the 5.5 image and running a scan on our ESXi hosts, 5 of 8 hosts are marked as incompatible. All 8 are identical hardware and all have the same version of ESXi installed ( 5.1.0 ).

I cannot find any differences in the configuration of any of the servers - anyone any idea why they might be showing as incompatible ?

Cheers.