Unable to control more than one object in gazebo with one node

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Hey,

I just tried to build up a changing world to train my Turtlebot3 in it. The idea is, that everytime when the robot spawns to (0, 0) again, parts of the layout and obstacles shall change. Here I try to switch positions and movement directions of my three obstacles when the Robot respawns. The problem is, only the first obstacle is moving. The other one don´t move at all. My question is, can I communicate with all obstacles from one node? In the following you can see my node code and a part of the .world file. Node:

#!/usr/bin/env python
import rospy
import time
import random
import math
from gazebo_msgs.msg import ModelState, ModelStates

class Combination():
    def __init__(self):
        self.pub_model = rospy.Publisher('gazebo/set_model_state', ModelState, queue_size=1)
        self.moving()

    def moving(self):
        state = 0
        while not rospy.is_shutdown():
            model = rospy.wait_for_message('gazebo/model_states', ModelStates)


            for i in range(len(model.name)):
                if model.name[i] == 'turtlebot3_burger':
                    turtlebot_pos= ModelState()
                    turtlebot_pos.model_name=model.name[i]
                    turtlebot_pos.pose = model.pose[i]

                    if abs(turtlebot_pos.pose.position.y)<0.01  and abs(turtlebot_pos.pose.position.x)<0.02:
                        for i in range(len(model.name)):
                            if model.name[i] == 'obstacle_1':
                                a = random.randint(1,2)
                                obstacle_1 = ModelState()
                                obstacle_1.model_name = model.name[i]
                                obstacle_1.pose = model.pose[i]
                                obstacle_1.twist = model.twist[i]
                                if a==1:
                                    obstacle_1.pose.position.x=2.5
                                    obstacle_1.pose.position.y=2.5
                                    if obstacle_1.pose.position.y > 2.2:
                                        obstacle_1.twist.linear.y = -0.25
                                    elif obstacle_1.pose.position.y < 2.2 and obstacle_1.pose.position.y > -1.2 and obstacle_1.twist.linear.y <0:
                                        obstacle_1.twist.linear.y = -0.25
                                    elif obstacle_1.pose.position.y < -1.2:
                                        obstacle_1.twist.linear.y = 0.25
                                    elif obstacle_1.pose.position.y < 2.2 and obstacle_1.pose.position.y > -1.2 and obstacle_1.twist.linear.y >0:
                                        obstacle_1.twist.linear.y = 0.25
                                if a==2:
                                    obstacle_1.pose.position.x=2.5
                                    obstacle_1.pose.position.y=2.5
                                    if obstacle_1.pose.position.x > 2.2:
                                        obstacle_1.twist.linear.x = -0.25
                                    elif obstacle_1.pose.position.x < 2.2 and obstacle_1.pose.position.x > 0 and obstacle_1.twist.linear.x <0:
                                        obstacle_1.twist.linear.x = -0.25
                                    elif obstacle_1.pose.position.x < 0:
                                        obstacle_1.twist.linear.x = 0.25
                                    elif obstacle_1.pose.position.x < 2.2 and obstacle_1.pose.position.x > 0 and obstacle_1.twist.linear.x >0:
                                        obstacle_1.twist.linear.x = 0.25

                            if model.name[i] == 'obstacle_2':
                                b = random.randint(1,2)
                                obstacle_2 = ModelState()
                                obstacle_2.model_name = model.name[i]
                                obstacle_2.pose = model.pose[i]
                                obstacle_2.twist = model.twist[i]
                                if obstacle_2.pose.position.x > 2.5:
                                    obstacle_2.twist.linear.x = -0.25
                                elif obstacle_2.pose.position.x < 2.5 and obstacle_2.pose.position.x > -0.5 and obstacle_2.twist.linear.x <0:
                                    obstacle_2.twist.linear.x = -0.25
                                elif obstacle_2.pse.position.x < -0.5:
                                    obstacle_2.twist.linear.x = 0.25
                                elif obstacle_2.pose.position.x < 2.5 and obstacle_2.pose.position.x > -0.5 and ...