High, constant training loss with CNN
I suspect you are facing a dying ReLU problem.  Check the gradients for each layer and see if they are starting to become 0.  Due to the way backpropagation works and a simple application of the chain rule, once a gradient is 0, it ceases to contribute to the model.  Therefore, if you're model is stuck then it's likely that a significant number of your neurons are now dead.

Try using a LeakyRelu or Maxout and see if that helps.