Welcome to GLO!
I'm not sure how Yast2 goes about installing the kernel sources in Suse, because I do everything from the command line, but if it's like any other kernal install you'll need to reboot. It may have succesfully created the new boot image, you're just still booted into the older kernel.
Rebuilding a kernel is one of the few times when you'll need to reboot your linux machine, but when you think about why, it makes sense.
Just my thoughts, but I'm sure someone else will have some more intuitive suggestions.