Hi xChiko,
> Clients are scheduled to update during the weekend only.
I very strongly recommend that they update at least daily. Once a week was OK back in 1995. It will get your network pwned here in 2012. &: )
The first thing I would do to troubleshoot is check which clients update and which do not, Chances are all the SEP 11 clients are OK, but LUA has not been configured to download and distribute SEP 12.1 materials. (or vice-versa)
Final recommendation: upgrade!! Al SEP 12.1 clients should be on 12.1 RU1 MP1. All SEP 11 should be on SEP 11 RU7 MP2. LUA must be upgraded to 2.3.1. Earlier versions have known vulnerabilities.
Hope this helps!!