Cannot achieve good result while Transfer Learning CIFAR-10 on ResNet50 - Keras

I'm trying to Transfer Learn ResNet50 for image classification of the CIFAR-10 dataset.

It's stated in the original paper and also ResNet50 documentation on keras.io that the ResNet should have a minimum input shape of 32x32. But I cannot achieve any good results.

Here I have created and compiled the sequential model:

model = Sequential()
model.add(ResNet50(include_top=False, weights='imagenet', input_shape=(32,32,3)))
model.add(Flatten())
model.add(BatchNormalization())
model.add(Dense(128, activation='relu'))                                        #Dense Layer
model.add(Dropout(0.5))                                                         #Dropout
model.add(Dense(10, activation='softmax'))                                      #Output Layer
model.layers[0].trainable = False                                               #Set ResNet as NOT trainable
model.summary()
model.compile(loss='categorical_crossentropy', optimizer='adam', metrics=['accuracy'])

and here I have loaded the CIFAR-10 dataset:

(x_train, y_train), (x_test, y_test) = cifar10.load_data()    #Load dataset
num_classes = len(np.unique(y_test))

(x_train, x_valid) = x_train[5000:],x_train[:5000]            #set 5000 validation data
(y_train, y_valid) = y_train[5000:],y_train[:5000]

y_train = to_categorical(y_train,num_classes)                 #Convert to one-hot
y_test  = to_categorical(y_test,num_classes)
y_valid = to_categorical(y_valid,num_classes)

Note that on the keras.io website documentation for the resnet50.preprocess_input, it is stated that the input data has 0-255 range. So then to preprocess data I used:

x_train = preprocess_input(x_train.copy())                  #preprocess training images for the resnet50
x_test  = preprocess_input(x_test.copy())                   #preprocess test images for the resnet50
x_valid = preprocess_input(x_valid.copy())                  #preprocess validation images for the resnet50

And here's the fitting section:

ModelHistory = model.fit(x_train, y_train,
                         batch_size=32,
                         epochs=10, verbose=0,
                         validation_data=(x_valid,y_valid),
                         callbacks=[earlystop])                     #Use Early Stopping

But the result I get is terrible even after ~20 epochs (~67% testing accuracy)

Where am I doing wrong?