validation accuracy is not increasing training ResNet50

Solution 1:

It was happening because i was just directly adding the fully-connected layers without training it first, as mentioned in Blog of keras, https://blog.keras.io/building-powerful-image-classification-models-using-very-little-data.html

in order to perform fine-tuning, all layers should start with properly trained weights: for instance you should not slap a randomly initialized fully-connected network on top of a pre-trained convolutional base. This is because the large gradient updates triggered by the randomly initialized weights would wreck the learned weights in the convolutional base. In our case this is why we first train the top-level classifier, and only then start fine-tuning convolutional weights alongside it.

so answer is first train the top-model separately, then create a new model having ResNet50 model with its weight, with top-model and its weights on top of resnet model(base-model), then train it first by freezing the base-model(ResNet50) and the with the last layer of the base-model.