Here is a more systematic approach to investigate on your issue rather than just some spare Q&A's:
1. there no need to break the replication, just disable the content replication to go back to the problematic scenario
2. in the problematic SEPM console > admin > servers, click on "Download LiveUpdate Content", what is the result of this action?
3. once it is completed, on the same SEPM console panel, click "Show LiveUpdate Downloads", are them up-to-date?
Possible resulting scenarios are:
1. failure during the download task, hence connectivity issue to our LU Servers
2. failure in publishing the latest content (i.e. the download is OK but "Show LU Downloads" does not reflect it) meaning there's a problem in the SEPM installation or with its DB
3. clients do not get the latest definitions (despite listed in the "show LU downloads") hence a communication issue between clients and SEPM
There might be some other corner cases but let's first focus on most common issues.