Issue while updating Managed MetaData programaticaly using C#

I had a sharepoint Library with versioning enabled. With C# code i am trying to update list.

When first time i create a list all the data get updated properly.

After that i am trying to update the Managed Meta data based on customer input. This timeManaged meta data is not updating.

I am using same code block both the scenario.

To assign value:   argDestItem[destField.Id] = argSourceItem[srcField.Id];

To update the sharepoint list: item.Update();

In second scenario its not working.

Any idea what should be done.

PS: This scenario is working when version is not enabled in Sharepoint. With versioning enabled its not working