[ubuntu] Missing new vmlinuz-x.xx.x-xx-generic.efi.signed after kernel update

I have installed Ubuntu 14.04.4 on a PC with UEFI and GPT, choosing the LVM option.
After installation, I could see in the /boot directory that the installed keel version was 3.19.0-51 and that there were also several other files with the same version.

A few days later I updated the system and, together with other package updates, also the keel got updated to the version 3.19.0-56 .
Looking again at the /boot directory, I could see that there was a new set of files with version 3.19.0-56, except for the generic.efi.signed that was missing:

-rw-r--r-- 1 root root 1271993 Feb 26 23:32 abi-3.19.0-51-generic
-rw-r--r-- 1 root root 1272130 Mar 11 13:10 abi-3.19.0-56-generic
-rw-r--r-- 1 root root 177808 Feb 26 23:32 config-3.19.0-51-generic
-rw-r--r-- 1 root root 177808 Mar 11 13:10 config-3.19.0-56-generic
drwxr-xr-x 3 root root 4096 Jan 1 1970 efi/
drwxr-xr-x 6 root root 1024 Mar 16 08:17 grub/
-rw-r--r-- 1 root root 20619855 Mar 7 13:35 initrd.img-3.19.0-51-generic
-rw-r--r-- 1 root root 21025201 Mar 14 16:25 initrd.img-3.19.0-56-generic
drwx------ 2 root root 12288 Mar 4 10:57 lost+found/
-rw-r--r-- 1 root root 176500 Mar 12 2014 memtest86+.bin
-rw-r--r-- 1 root root 178176 Mar 12 2014 memtest86+.elf
-rw-r--r-- 1 root root 178680 Mar 12 2014 memtest86+_multiboot.bin
-rw------- 1 root root 3630758 Feb 26 23:32 System.map-3.19.0-51-generic
-rw------- 1 root root 3631082 Mar 11 13:10 System.map-3.19.0-56-generic
-rw------- 1 root root 6575024 Feb 26 23:32 vmlinuz-3.19.0-51-generic
-rw------- 1 root root 6576952 Mar 4 10:59 vmlinuz-3.19.0-51-generic.efi.signed
-rw------- 1 root root 6579344 Mar 11 13:10 vmlinuz-3.19.0-56-generic

I ask myself why the file vmlinuz-3.19.0-56-generic.efi.signed has not been generated: maybe is something wrong in the configuration ?

Surfing the web for a solution, I could see that other people have always a generic.efi.signed file for every keel version in the /boot directory.

Currently I can boot the PC in EFI mode with the new keel, even if the corresponding generic.efi.signed file is missing. Why?

How can I fix my system, letting generate a new generic.efi.signed file when a keel is updated ?

Many thanks!