Well I have just figured out why some of the machines that I use had stopped picking up updates. When I look at the list of systems on the RedHat Network they had a list of updates that they hadn’t picked up but when I logged into the machines and ran

yum update

it said there were no updates. After trying a lot of things on one of the systems that I was free to test with I still couldn’t find out what was going on. Eventually I discovered that yum had corrupted it’s cache and so it thought that its list of packages was up to date when it wasn’t. The solution was quite easy after that

yum clean all

yum update

I know, I should have tried that at the start.