My DQN is stuck and can't see where the problem is

Question

I'm trying to replicate the DeepMind paper results, so I implemented my own DQN. I left it training for more than 4 million frames (more than 2000 episodes) on SpaceInvaders-v4 (OpenAI-Gym) and it couldn't finish a full episode. I tried two different learning rates (0.0001 and 0.00125) and seems to work better with 0.0001, but the median score never raises above 200. I'm using a double DQN. Here is my code and some photos of the graphs I'm getting each session. Between sessions I'm saving the network weights; I'm updating the target network every 1000 steps. I can't see if I'm doing something wrong, so any help would be appreciated. I'm using the same CNN construction as the DQN paper.

Here's the action selection function; it uses a batch of 4 80x80 processed experiences in grayscale to select the action (s_batch means for state batch):

    def action_selection(self, s_batch):
        action_values = self.parallel_model.predict(s_batch)
        best_action = np.argmax(action_values)
        best_action_value = action_values[0, best_action]
        random_value = np.random.random()

        if random_value < AI.epsilon:
            best_action = np.random.randint(0, AI.action_size)
        return best_action, best_action_value

Here is my training function. It uses the past experiences as training; I tried to implement that if it lose any life, it wouldn't get any extra rewards, so in theory, the agent would try to not die:

    def training(self, replay_s_batch, replay_ns_batch):
        Q_values = []
        batch_size = len(AI.replay_s_batch)
        Q_values = np.zeros((batch_size, AI.action_size))

        for m in range(batch_size):

            Q_values[m] = self.parallel_model.predict(AI.replay_s_batch[m].reshape(AI.batch_shape))
            new_Q = self.parallel_target_model.predict(AI.replay_ns_batch[m].reshape(AI.batch_shape))
            Q_values[m, [item[0] for item in AI.replay_a_batch][m]] = AI.replay_r_batch[m]

            if np.all(AI.replay_d_batch[m] == True):
                Q_values[m, [item[0] for item in AI.replay_a_batch][m]] = AI.gamma * np.max(new_Q)    

        if lives == 0:
            loss = self.parallel_model.fit(np.asarray(AI.replay_s_batch).reshape(batch_size,80,80,4), Q_values, batch_size=batch_size, verbose=0)

        if AI.epsilon > AI.final_epsilon:
            AI.epsilon -= (AI.initial_epsilon-AI.final_epsilon)/AI.epsilon_decay

replay_s_batch it's a batch of (batch_size) experience replay states (packs of 4 experiences), and replay_ns_batch it's full of 4 next states. The batch size is 32.

And here are some results, after training:
In blue, the loss (I think it's correct; it's near-zero). Red dots are the different match scores (as you can see, it does sometimes really good matches). In green, the median (near 190 in this training, with learning rate = 0.0001) Here is the last training, with lr = 0.00125; the results are worse (it's median it's about 160). Anyway the line it's almost straight, I don't see any variation in any case. So anyone can point me to the right direction? I tried a similar approach with pendulum and it worked properly. I know that with Atari games it takes more time but a week or so I think it's enough, and it seems to be stuck. In case someone need to see another part of my code just tell me.

Edit: With the suggestions provided, I modified the action_selection function. Here it is:

def action_selection(self, s_batch):
    if np.random.rand() < AI.epsilon:
        best_action = env.action_space.sample()
    else:
        action_values = self.parallel_model.predict(s_batch)
        best_action = np.argmax(action_values[0])
    return best_action

To clarify my last edit: with action_values you get the q values; with best_action you get the action which corresponds to the max q value. Should I return that or just the max q value?

Answer 1

After some research and reading this post, I see where my problem was: I was introducing a full consecutive batch of experiences, selected randomly, yes, but the experiences in the batch were consecutives. After redoing my experience selection method, my DQN is actually working and has reached about +200 points after 400000 experiences (about 500 episodes; only 2-3 hours or training!). Before I couldn't reach that score after days of training. I'll let it train to see if there are something I can improve. Thanks to everyone who tried to help me! I let this answer here just in case someone has the same problem as me.