Why is apt no longer updating the kernel?

Solution 1:

You have probably removed linux metapackage by accident. If so, that should be fixed by:

sudo apt-get install linux

Further explanation:

Because of technical reasons it's not wise for the kernels to be upgraded and replaced with the new one (i.e. let's say if the new one fails you may still boot into the old one).
Thus they are packaged under the names like linux-image-3.2.0-24-generic.

So, if you upgrade the system, technically there is no update to this version! Unless you install the linux metapackage which always depends on the latest kernel, meaning that whenever an update to the kernel is available, linux will change version, and it will depend on the newer kernel, and thus the new kernel will be automatically installed.

Remember that your older kernel won't be removed this way, which is a good thing. If you find anything odd you may boot into the old one (grub -> Previous versions or something like that) and remove the new kernel.