Using revision r2554 of Exaile, there are two tracks (so far) in my whole collection which cannot be added to the playlist individually. Neither clicking on them, nor drag-and-dropping them to the playlist, nor "Append to Current" works. If the problematic tracks are selected along with some working files, they also do not get added while the flawless tracks do. There are two ways to get them added to playlist: 1. add the whole album or 2. append from file manager.
I do not notice a difference when changing the collection view or using search, as in bug #427803. It is possible that the reporter has some affected files too. However, it may also be totally unrelated to this problem.
You can try it for yourself, the affected tracks are track 2 and 3 from this Jamendo album. The audio format does not matter, it happens with both MP3 and OGG: http://www.jamendo.com/de/album/35508
This is reproducible on any track with two consecutive spaces in the title (haven't tried if it affects artist/album too).
For example, track #2 is "Effata (Get open)", and track #3 is "Goodbye ".