Sometimes BlueJ doesn't compile correctly. The status bar indicates that the compiler is working, but it never seems to finish. I've noticed the problem three times now.
The problem seems to occur when code is copied inside the editor, or when the Ctrl-Space function is used. The third time there was some syntax errors that the compiler didn't seem to find.
I've had this problem in version 3.3 and previous. Haven't tested the latest, but the readme didn't say anything about fixing such a bug.