After searching and searching online I've found the answer to this problem and the solution is quite simple.
Apparently it's because the graphic drivers don't load quickly enough during the booting sequence so by the time the graphic drivers are ready booting has almost finished and thus either no Plymouth screen is shown or a very quick fast of it is seen.
So the solution I found and it works is to add a simple line to the following file:
gksu gedit /etc/initramfs-tools/conf.d/splash
then add to it:
Save the file, and then run:
sudo update-initramfs -u -k all
This command will update the initial ramdisk containing the drivers needed by the kernel at boot time. The parameter
-k all updates all of them if you have more than one kernel installed. Reboot and all is well.
This works in ubuntu 12.04 too.
Hope this helps anyone who is missing the lovely Ubuntu boot screen.