gameObject disapearing and falling through terain

So I'm newbie in Unity, I am building my first platformer 2D, as help I am using a 2D kit, for some reason the enemies are falling through the map and disapearing when I run the game. I used the tilemap system to build the terain and background... I have a flying type of Enemy that's acting normally but the other just disapears and I can see the Y position sinking when I launch the game. did anyone run into this problem and can help me, thanks. the gamekit is this one : https://github.com/atmosgames/SuperSimple2DKit