You'll need to export the master repository key from the first server, and import it into the second. (This is because the second server will reject the content that's signed by the first server, which is probably what's happening here.) See page 71 in the ePO 4 user guide.
Warning - make sure that you are only replicating content updates, like DATs and engines, to the repository that the second ePO server is pulling from. Otherwise you run the risk of all the second server's machines trying to talk to the first server, which you don't want
Brilliant, that's done the trick.
We've recently migrated over to a new main ePO server so that explains why it hasn't worked since.
Thanks for you help.