Not receiving messages from endpoints after server migration

Hi,

Recently I completed a server to server migration of the Enterprise Console from Server 2003 32 bit to 2012 R2 64 bit as per this guide: https://www.sophos.com/en-us/support/knowledgebase/28276.aspx 

Everything looks to have gone well, however, a few days down the line and around half of the endpoints are reporting "Update to date: not since <2 days ago>". This time and date match the "Last message received from computer" column but when I look at the local computers in question, in Endpoint Security and Control I can see that they are recieving updates from their local SUM and are indeed up to date.

I'm not quite sure which logs to check here apart from the local updating log which shows everything is ok. 

I presume that I didn't allow enough time for one of the policies to propigate to all of the endpoints before decomissioning the old server.

Any help would be much appreciated!

Thanks