update kernel: Install versus update

Hello,
I am running 3 Fedora Core 6 systems on 3 PCs, all configured quite 
similarly. On two of the machines yum removes the old kernel, when it 
installs a new one, on the third one it doesn't remove the old kernel.
(Unfortunately that's just the one with the smallest /boot partition, so 
I have to deinstall the old kernels by hand, but it's certainly not a 
big issue.)
Is there any switch or whatever to tell the third machine to behave like 
the other ones? I couldn't find any difference in the configuration files.
Alois