CocosSharp trying to make 2D game, addChild not working as planned

Question

I'm trying to make a simple game where there's a zombie horde coming from the top of the screen to the bottom. I want that a new zombie appears every 5 seconds, and it does, but every time a new zombie appears the previous one stops moving and the collision doesn't work on that one. Can someone help me understand this behaviour and what's the best way to make it work the way its supposed to? :) Here is my where I create the zombie:

private void CreateZombie()
{
    zombieSprite = new CCSprite ("zombie"); 
    zombieSprite.PositionX = CCRandom.GetRandomFloat (10, 600);
    zombieSprite.PositionY = 1055; 
    AddChild (zombieSprite);
}

and here's the code inside my gamelogic method:

void GameLogic (float frameTImeInSeconds) {
        zombieYVelocity += frameTImeInSeconds * -gravity; 
        zombieSprite.PositionY += zombieYVelocity * frameTImeInSeconds; 

        if (timer % 5 == 0) {

            CreateZombie ();
            zombieYVelocity = 0; 

        }   

    }

I attached a screenshot that shows whats happening Every 5 seconds when a new one is added the previous one stops, and the collision detection is no longer working with the ones that are stoped.