Hello WKLeong ,
try this on one machine go to client tab select the any group in which export communication settings name and save this file in shared location and name it as sylink.xml
from sepm delete the entry of machine on which you ll be replacing sylink file
how to replace sylink in 12.1 as below
http://www.symantec.com/business/support/index?page=content&id=TECH157585&actp=search&viewlocale=en_US&searchid=1321547580556
Simultaneously you can try to delete few clients from sepm and reboot them and check if they go green and report back to sepm by default they should
Try changing push to pull mode only for communication purposes once the clients are communicating change the mode back to push
Were the client machines cloned images ever ?
try below
http://www.symantec.com/business/support/index?page=content&id=TECH102815