boost log file not written to

I've been struggling with boost log for a while now - I got their simple example writing to a log file (http://boost-log.sourceforge.net/libs/log/example/doc/tutorial_file.cpp). However, when I try to copy that code into a 'Logger' class, I can't get it to write to the log file. I can see the file default.log get created, but there is nothing in it.

I'm on debian 7 64bit. Everything compiles fine - compile line is:

g++ -o build/Logger.o -c -std=c++11 -Wall -g -O0 -DBOOST_LOG_DYN_LINK -DDEBUG src/Logger.cpp
g++ -o build/logtest build/Logger.o -lboost_log -lboost_log_setup -lboost_date_time -lboost_thread -lboost_wave -lboost_regex -lboost_program_options

Here's my code:

Logger.cpp

/*
 * Logger.cpp
 *
 *  Created on: 2011-01-17
 *      Author: jarrett
 */

#include "Logger.h"

namespace logging = boost::log;
namespace sinks = boost::log::sinks;
namespace src = boost::log::sources;
namespace expr = boost::log::expressions;
namespace attrs = boost::log::attributes;
namespace keywords = boost::log::keywords;

namespace dhlogging {

Logger::Logger(std::string fileName)
{
    initialize(fileName);
}

Logger::Logger(Logger const&)
{
}

Logger::~Logger()
{

}

Logger* Logger::logger_ = nullptr;
Logger* Logger::getInstance(std::string logFile)
{
    if ( Logger::logger_ == nullptr ) {
        logging::add_file_log( logFile );

        logging::core::get()->set_filter
        (
            logging::trivial::severity >= logging::trivial::info
        );

        logging::add_common_attributes();
        Logger::logger_ = new Logger(logFile);
    }

    return Logger::logger_;
}

void Logger::initialize(std::string fileName)
{   
    BOOST_LOG(log_) << "Hello, World!";
    BOOST_LOG_SEV(log_, info) << "Hello, World2!";
}

void Logger::logInfo(std::string messa