I can define new abbreviations no problem: e.g.
:ab Lunix Linux
defines an "Lunix" as an abbreviation for "Linux", as confirmed by the list which appears when I execute :ab.
In a plain text file, such as an empty buffer, this expands correctly, but with some filetypes neither SPACE nor CTRL-] seem to work.
Is there a setting or mapping which might be getting messed up by some of my filetype plugins?
There's probably a really obvious answer to this, but I haven't found it hunting through the :help documentation yet.