Hi,
From the graph above, it looks like the loss(assuming train loss) is barely changing. Can you give some more insight about the problem? what is the dataset used, image size etc.
Meanwhile, you can do the following:

Check if the data is correct(valid ground truth, input to the network is correct).
Try to overfit on a small subset of data.
Try using a higher learning rate from the point where the loss stagnates.