MPU-6050 dmp with HMC5883L

I use a code download from github, the dmp part is from Jeff Rowberg .

The point is the compass part, it uses the EXT_SENS_DATA registers to read the compass data,but the FIFO overflow warning would come out every certain number time and reset the FIFO,after couples of warnings, the code stucks.

But the original DMP code doesn't have this problem ,so does any one know why the FIFO buffer keeps overflow??

setup{

  mpu.setI2CMasterModeEnabled(0);//0x6A master_mode disabled
  mpu.setI2CBypassEnabled(1);//Bypass Enabled

  Wire.beginTransmission(HMC5883L_DEFAULT_ADDRESS);
  Wire.write(0x02); 
  Wire.write(0x00);  // Set continuous mode
  Wire.endTransmission();
  delay(5);

  Wire.beginTransmission(HMC5883L_DEFAULT_ADDRESS);
  Wire.write(0x00);
  Wire.write(B00011000);  // 75Hz
  Wire.endTransmission();
  delay(5);

  mpu.setI2CBypassEnabled(0);

  // X axis word
  mpu.setSlaveAddress(0, HMC5883L_DEFAULT_ADDRESS | 0x80); // 0x80 turns 7th bit ON, according to datasheet, 7th bit controls Read/Write direction
  mpu.setSlaveRegister(0, HMC5883L_RA_DATAX_H);0x03
  mpu.setSlaveEnabled(0, true);
  mpu.setSlaveWordByteSwap(0, false);
  mpu.setSlaveWriteMode(0, false);
  mpu.setSlaveWordGroupOffset(0, false);
  mpu.setSlaveDataLength(0, 2);

  // Y axis word
  mpu.setSlaveAddress(1, HMC5883L_DEFAULT_ADDRESS | 0x80);
  mpu.setSlaveRegister(1, HMC5883L_RA_DATAY_H);0x05
  mpu.setSlaveEnabled(1, true);
  mpu.setSlaveWordByteSwap(1, false);
  mpu.setSlaveWriteMode(1, false);
  mpu.setSlaveWordGroupOffset(1, false);
  mpu.setSlaveDataLength(1, 2);

  // Z axis word
  mpu.setSlaveAddress(2, HMC5883L_DEFAULT_ADDRESS | 0x80);
  mpu.setSlaveRegister(2, HMC5883L_RA_DATAZ_H);0x07
  mpu.setSlaveEnabled(2, true);
  mpu.setSlaveWordByteSwap(2, false);
  mpu.setSlaveWriteMode(2, false);
  mpu.setSlaveWordGroupOffset(2, false);
  mpu.setSlaveDataLength(2, 2);

  mpu.setI2CMasterModeEnabled(1);


}
loop{

mx=mpu.getExternalSensorWord(0);
my=mpu.getExternalSensorWord(2);
mz=mpu.getExternalSensorWord(4);

}