Do you have the kernel meta-package installed ? The kernels rely on having the kernel meta-package installed to know when there is a kernel update.
Depending on what kernel and version of Ubuntu you have installed, you'll be looking for one of these...
The ones highlighted in bold are the usual ones most people need. Depending on what you have installed, it is usually one or the other.
linux-image-extra-virtual - Transitional package.
linux-image - Linux kernel image
linux-image-generic - Generic Linux kernel image
linux-image-generic-pae - Transitional package
linux-image-lowlatency - lowlatency Linux kernel image
linux-image-lowlatency-pae - lowlatency Linux kernel image
linux-image-server - Transitional package.
linux-image-virtual - This package will always depend on the latest minimal generic kernel image.
linux-virtual - Minimal Generic Linux kernel and headers
You can see what is installed using..
That is a small L after dpkg.
dpkg -l | grep linux-image