Unable to shrink transaction log file in SQL Server
The reason your process isn't working is that it's waiting to release the inactive portion of the log using a transaction log backup. Switching to Simple Recovery Model bypasses this issue.

If you really need to shrink down the transaction log, so that you can regrow it (to reduce the number of VLFs, for instance), this is the process:

1) Switch to Simple Recovery Model

2) Run a CHECKPOINT and DBCC DROPCLEANBUFFERS (just in case)

3) Shrink the log file

4) Switch back to Full Recovery Model

5) Grow the log in reasonable increments (I use 4000MB increments)

6) Set autogrowth to a value your I/O subsystem can handle

7) Run a full or differential backup to restart the log backup chain.