It is likely not shrinking because the active part of the log is at the end.

Run a few checkpoints and back up the log again and you should be able to shrink the log file.

Remember to shrink it all the way down to 0 and then extend it to the desired size.

If the desired size is greater than 4gb, then extend it in 4gb chunks (has to do with avoiding VLFs that are too large).

USE [<db_name>]

GO

CHECKPOINT

GO

DBCC SHRINKFILE (N'<db_name>_log' , 0)

GO

USE [master]

GO

ALTER DATABASE [<db_name>] MODIFY FILE ( NAME = N'<db_name>_log', SIZE = 4194304KB )

GO

ALTER DATABASE [<db_name>] MODIFY FILE ( NAME = N'<db_name>_log', SIZE = 8388608KB )

GO