Model weights not updated

Hi there. I’m trying to update the weights of multiple models at once where the optimizer is set as:
optimizer = torch.optim.SGD(list(netA.parameters()) + list(netB.parameters())+ list(netC.parameters()),
For the weight update, I performed loss.backward() and checked that all the parameter’s some_param.weight.grad is not None. I ran optimizer.step() and always find that only the parameters of netA are updated but not the rest. Any idea on what am I missing here? Thanks!

I found the problem. The other models are actually based on the first order gradient of the first model, hence the gradient becomes very small and are attenuated.