Adding more layers decreases accuracy I have my ANN trained on MNIST dataset. Hidden layer has 128 neurons and input layer has 784 neurons. This gave me an accuracy of 94%. However when I added one more layer with 64 neurons in each then the accuracy significantly reduced to 35%. What could be the reason behind this. 

Edit : Activation function : sigmoid. 521 epochs.
The reason is that by adding more layers, you've added more trainable parameter to your model. You have to train it more. You should consider that MNIST data set is a very easy-to-learn dataset. You can have to layers with much less number of neurons in each layer. Try $10$ neurons for each to facilitate the learning process. You can reach to $100%$ accuracy.