I am trying to add a .a file to my Xcode project, but it is ignored by SVN and therefore I cannot add it to the repository. Is there any clue what could be causing?
Yes, you have to do it manually. Just go to it in the console and run the svn add file_name.a command and it should work.
SVN has a global ignore list. By default, it includes masks for most popular types of generated files .. a is one of them.
SVN, miscellany/global-ignores. ~/.subversion/config. Finder .