I can think of two reasons:

1 - there is an active transaction. you cannot truncate the log past the oldest active transaction. Use DBCC OPENTRAN() to find the oldest open transaction in the log.

2 - the active virtual-log is the last one in the file. you will not be able to shink the file until the active virtual-log cycles back to the front. Please refer to the topic Shrinking the Transaction Log in the BOL for more info. Use DBCC LOGINFO(databasename) to see how your transaction log is divided into virtual logs. A 2 in the Status column indicates that that virtual log is active.

-Mike


>> Truncation does not physically reduce the size of the log file.
>> For that, run
>>
>> USE myDatabaseName
>> DBCC SHRINKFILE (myDatabaseName_Log)
>>
>> The log file retains all transactions executed on your database unless you > set options to control that, that's why it may be bigger than the actual
>> database.
>
>I did with success the command DBCC SHRINKFILE to reduce my log file many times in the past (every month), and now it is unsuccessful. I do not understand why suddenly it do not works. Someone can explain to me why??