This can happen quite often and be caused by quite a few different issues.
If the machines are reporting in ePO correctly then its not a McAfee Agent issue, so reinstalling this will not help in this situation. You can reinstall the AV product, which will probably fix your issue but is always the last method you would like to use.
You can use ePOMVT to diagnose and fix any anomylies with the AV product,
Or you can import an SDAT and push this to the machine, I often fine that this can have a positive affect on the machines (although I don't know why this does)
I use some other methods but they get a little bit more complicated depending on your environment ie Login Scripts fixes etc.
In the first instance, you need to check the agent_machinename.log and mcscript.log on the affected client machines - these should give us an idea of where the problem is.