log4net doesn't log when built in RELEASE mode
If you use attributes to configure log4net then the order by which assemblies are loaded may determine whether you attributes are used or not. Assembly load order may be different in DEBUG and RELEASE mode.
As stated in the manual the attribute will only be read for the first assembly that tries to use log4net. So it is important that you obtain your ILog instance as early as possible.
For a command line application "as early as possible" probably is the class holding the Main method, for a Web-Application it would be your Global.asax class and for a Windows Service it would be the class deriving from ServiceBase.