using – Streamwriter error is written in the file it says it cannot access

I have a simple routine in a C# Console Application That writes log file entries. Today, an odd message appeared in the log.

The process cannot access the file 'C:Development-IPLLogfilesLogFile20212.txt' because it is being used by another process. 

This is bizarre – the message is written in the file it says it cannot access.

The code looks like this:

public static void WriteLog(string message)
    {
        var requiredPath = Path.GetFullPath(Path.Combine(AppDomain.CurrentDomain.BaseDirectory, "..\..\"));

        // use Logfileyyyyww where yyyy is year & ww is week number

        DateTime dateTime = DateTime.Now;
        int year = dateTime.Year;
        int weekno = WeekNo.GetIso8601WeekOfYear(dateTime);

        string filePath = requiredPath + "\Logfiles\LogFile" + year.ToString() + weekno.ToString() + ".txt";
        using (StreamWriter writer = new StreamWriter(filePath, true))
        {
            if (message != null)
            {
                writer.WriteLine("Date : " + DateTime.Now.ToString() + ": " + message);
            }
        }
    }

I would have thought that the Using statement would ensure that this resource is closed & disposed each time it is used.