Due to a known bug in Fedora 20 with some selinux updates I recognized a connected problem on my machine. While updating some packages I found this message:
Security: kernel-3.12.8-300.fc20.x86_64 is an installed security update Security: kernel-3.12.7-300.fc20.x86_64 is the currently running version
To fix this and get the latest kernel visible in grub and set as default I needed to reinstall it by running:
yum remove kernel-3.12.8-300.fc20
again. After this procedure the new kernel was available.