dpkg has a very useful feature where if you delete a conffile (pretty
much everything under /etc and a few other files) it isn’t
replaced when you upgrade the package[0]. This behaviour was
confusing me for a while until I realised what was happening. I was
attempting to reinstall a package to get the default configuration
files back that had been accidentally deleted, but no matter what I
tried, the files didn’t exist after running dpkg. Once I
figured out that dpkg had this behaviour the solution was
simple; use the --force-confmiss command line argument.

root@quux:~# dpkg --force-confmiss -i /tmp/foo_2.0.0-build.14_all.deb
(Reading database ... 33418 files and directories currently installed.)
Preparing to replace foo 2.0.0-build.14 (using .../foo_2.0.0-build.14_all.deb) ...
Unpacking replacement foo ...
Setting up foo (2.0.0-build.14) ...

Configuration file `/etc/foo/foo.xml', does not exist on system.
Installing new config file as you request.
[0] If the file didn’t exist in
the previously installed version, it is installed, so you get new
configuration files.