Linear layer params not updating

I’ve been implementing a data fusion model, and it mostly works, but in one of the modules I am having trouble with the parameters of a linear layer not updating. I’m guessing it’s something in my intermediate calculations, but I can’t seem to figure out what.
Here is my init and forward implementation:

class WeightedCombinationModule(nn.Module):
    def __init__(self, in_size, pool_size, hidden_size=64, 
                    rnn_layers=2, dropout=0.7, a=9.0, b=0.01, c=10.0):
        super(WeightedCombinationModule, self).__init__()
        # create class variables
        # pooling layer
        self.Pool = nn.MaxPool2d((1, self.pool_size), self.pool_size//2)
        # quality weight parameters
        wc_h = 1
        wc_w = (self.in_size[-1] - (self.pool_size-1)-1) // (self.pool_size//2) + 1

        self.lin = nn.Linear(wc_h*wc_w*in_size[1], 1)

        # RNN 
        self.gru = nn.GRU(wc_w, self.hidden_size, self.rnn_layers, batch_first=True)

    def forward(self, Sn):     
        Fn = torch.stack([self.Pool(s.squeeze(1)) for s in Sn.split(1,1)], 1)

        # encoding vector
        un = torch.flatten(Fn, 2, 4)
        # quality weights 
        inter = self.lin(un).squeeze(2)
        en = torch.div(inter, un.size(2))
        a_tildas = self.a / (1+torch.exp(-en/self.b)) + self.c

        a_sum = torch.sum(a_tildas, 1)
        alphas = a_tildas / a_sum.unsqueeze(1)
        a_temp = alphas.view(*alphas.size(), 1, 1, 1)
        C = a_temp * Fn
        C = torch.sum(C, 1).squeeze(2)
        # rnn
        output, _ = self.gru(C) # (batch, seq-len, hidden-size)
        r_wc = output.sum(2)  # (batch, seq-len)
        return r_wc

Edit: For clarity, here is a sample of one step’s gradients. You can see that the params of the linear layer have no gradient for some reason.

Could you print the gradients directly from the self.lin layer?
If the computation graph is detached somewhere (which I cannot see in your code, as it looks fine), you shouldn’t be able to see any values in print(model.lin.weight.grad) but it should instead print a None value.