There have been several times when i cannot get a transaction log to shrink no matter how many times I back up the transaction log and have set the recovery mode to Simple. You can reset the transaction log so that it truncates by running the T-SQL listed below. Only do this as a last resort since it may affect your ability to restore to a point in time. As always, backup your database and the transaction log. After you run the code, back up the database and try shrinking it again. In some cases you may have to shrink it a few times.
EXEC sp_repldone @xactid = NULL, @xact_segno = NULL, @numtrans = 0, @time = 0, @reset = 1