How do I list the packages that are recommended by my current set of installed packages, but for whatever reason weren't installed?
For a command-line list, From:
To complete your original question:
This can sometimes happen if a package B is installed as a recommendation when you install package A; package B's own recommends typically do not get installed.
1. Listing uninstalled Recommends from the terminal:
2. Listing uninstalled Recommends and Suggests from Synaptic: