I should also note that I'm also in the process of upgrading the McAfee Agents from 188.8.131.5222 to 184.108.40.20662, but I really don't think that it's the cause.
There's a number of things that this could be, but not being able to access the client machines will make diagnosis a little complicated.
In the first instance, you could try this - but *not* if you are using EEPC on the affected machine:
Delete an affected machine from ePO, but do not select the "remove agent" option. When the machine communicates again it will create a new entry in ePO - once this has happened, are the properties reported correctly?