To check if your model is not learning you need to check the requires_grad attribute for model weights and make sure the optimizer is correct. To check the requires_grad:
for param in my_model.parameters():
    print(param.requires_grad)

Training crash? You need to remove your previous models from GPU memory. This can be bit tricky. If you are running regular python script, then the following may work
del my_model
torch.cuda.empty_cache()

#Maybe run gc.collect()