Why is my kernel image not upgraded?

гm using Убунту Килин, when I type:

uname -r

I got:

3.16.0-53-generic

And in my/boot:

abi-3.16.0-52-generic     initrd.img-3.16.0-52-generic  System.map-3.16.0-53-generic
abi-3.16.0-53-generic     initrd.img-3.16.0-53-generic  System.map-3.16.0-55-generic
abi-3.16.0-55-generic     initrd.img-3.16.0-55-generic  System.map-3.16.0-56-generic
abi-3.16.0-56-generic     initrd.img-3.16.0-56-generic  vmlinuz-3.16.0-52-generic
config-3.16.0-52-generic  lost+found                    vmlinuz-3.16.0-53-generic
config-3.16.0-53-generic  memtest86+.bin                vmlinuz-3.16.0-55-generic
config-3.16.0-55-generic  memtest86+.elf                vmlinuz-3.16.0-56-generic
config-3.16.0-56-generic  memtest86+_multiboot.bin
grub                      System.map-3.16.0-52-generic

when I type apt-get upgrade, I got:

Reading package lists... Done
Building dependency tree       
Reading state information... Done
Calculating upgrade... Done
The following packages have been kept back:
linux-generic-lts-utopic linux-headers-generic-lts-utopic
linux-image-generic-lts-utopic
0 upgraded, 0 newly installed, 0 to remove and 3 not upgraded.

How хан I get my Ubuntu upgraded to 3.16.0-56?
Why is it not automatically getting upgraded? Thank you.