kernel upgrade held back

[R Kimber]

If I do:

sudo apt-get update
sudo apt-get upgrade

I get:

The following packages have been kept back:
  linux-headers-generic linux-signed-generic linux-signed-image-generic

What might be the reason for this?  As far as I know there are no broken
packages.

Also, the software updater progress window just closes when it tries to
install these security upgrades.

- Richard
-- 
Richard Kimber