Not sure- but if you are restoring from VMs or using snapshots the sequence number may be the problem.
If you open the server.log on the epo server then get the client to send props back to the server (run "c:\Program Files\McAfee\Common Framework\cmdagent .exe" /s etc or whatever dir the agent is installed to) see if the server rejects it due to a previous sequence number.
This is most likely caused by the sequence number checking in ePO. Please see KB65611 for more information - this is still applicable even though it refers to ePO 4. (I'll get it updated.)
You may need to implement the ConnectionsRequireValidSequenceNumber=0 option in the server.ini, although we don't generally recommend this.
How can I push those system connecting again ?