1. Add a few more balls, and update the objBall→Step event:
if (place_meeting(x + hspeed, y, objWall)){
hspeed *= -1;
}
if (place_meeting(x, y + vspeed, objWall)){
vspeed *= -1;
}
if (place_meeting(x + hspeed, y + vspeed, objBall)){
move_bounce_all(true);
}
2. You may need to update the collision mask:

3. Try alternative
step
event for objBall (can remove create event code):
switch(keyboard_key){
case vk_left:
if not(place_meeting(x - 3, y, objWall)){
x -= 3;
}
break;
case vk_right:
if not(place_meeting(x + 3, y, objWall)){
x += 3;
}
break;
}