Not sure what 1 in 1 out LSTM is, you want to have 1 of input and output features, while using 100 for hidden size? I think the standard practice is to add a Linear layer on top of the LSTM. If the network learns with hidden states of size 1, but doesn’t with that of size 100, it might mean that the optimization task becomes too complex for a simple task that you’re training the network to perform. Can’t really help you a lot, it depends on a lot of factors.