Various reasons may cause SQL Server to get in a rut and not empty the transaction log of a database. In my case, our database backups were failing without our knowledge for several weeks, so the backups were never successful, and the transaction logs of a few databases grew so large that the backup process would still not clear out the transaction log. In one case, we had a 187MB database with a 37GB transaction log!
The insanity had to stop! A handful of databases like this would put us over the top on that particular server's hard drive storage.
The SQL Server GUI for shrinking the database rendered no effect, and even using the DBCC SHRINKFILE command was not working.
The key, as explained by Pinal Dave, is to run the SHRINKFILE command twice, with an explicit backup log truncation in between both runs. This code here will get you up and running:
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DBCC SHRINKFILE("MyDatabase_Log", 1)
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BACKUP LOG MyDatabase WITH TRUNCATE_ONLY
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DBCC SHRINKFILE("MyDatabase_Log", 1)
This freed up dozens of gigabytes on our server.