ok - similar thing happened to me.....
download the 'slinkdrop' tool - you're supposed to have to open a support case with symantec for this, but there's plenty of people sharing this on the net...I can also email you a copy if you like.
then in the SEPM console, find the group that these clients are in...right click the group and select 'export communication settings'
save the xml file to the sylinkdrop folder....run the sylinkdrop.exe tool on the machine effected and select the exported communication file - it should resume communication again with the SEPM server