There is a problem about convergence

When i train my image use this Code,there is a problem about convergence: Training results shows that the 'total loss' is fluctuate around 2 ,and did not decrease anymore; Second, the 'loss_box' always decay to zero and it will rise a little only; Third, the training is always cut off, no reason; Fourth, the candidate boxes are generally small than the fact; Fifth, when i run the demo there are no object has been detected. So, what happen to my project, please reply to me as soon as possible,thank U!