The machine was in the correct OU, there were no filtering security groups. DNS checked out in both forward and reverse. DHCP also checked out.
the third possibility is that the PC isn't even realising that it should be picking up group policies at all - you then have to start checking if the machine is in the right OU, are you filtering on security groups and the machines not in the right group (or are there other "wrong" filters). DNS issues are possible here (although unlikely if the machine is just using DHCP and the others are all working)