How to find out why packages are "held back"

[stan]

I have one machine at work on Gusty thta seems to not to wnat to upgrade some packages:

Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following packages have been kept back:
  linux-headers-generic linux-image-generic linux-restricted-modules-generic
  openssh-client openssh-server ssl-cert

How can I fix this?

-- 
One of the main causes of the fall of the roman empire was that, lacking
zero, they had no way to indicate successful termination of their C
programs.