Eclipse has just stopped compiling changes to any of my projects, not sure why. I still have build automatically selected. Seeing as it affects all my projects I doubt that is the issue.
Solution
Well, it depends.
Imported project from SVN/GIT? - Make sure the classpath and buildpath configurations are correct
Make sure your projects are Java projects
Make sure that your Source folders are on build path
Try restarting Eclipse or your machine from time to time
Prefs -> Java Compiler -> Use default compliance settings