Log file shrink attempts are futile
The documentation to dbcc shrinkfile says:


  Because a log file can only be shrunk to a virtual log file boundary, shrinking a log file to a size smaller than the size of a virtual log file might not be possible, even if it is not being used. The size of the virtual log file is chosen dynamically by the Database Engine when log files are created or extended. 


So it isn't possible to shrink the logfile to an arbitrary small size. To check your virtual log files, you can use


  DBCC LOGINFO


There you can see the virtual log files and their size. Probably you hit the virtual log file size.