Moving rigidbody with addforce, velocity or position causes another object not to collide anymore.

I have 4 objects.

1. A UI hand witch is moved by a players hand position using a depth camera.

2. A 3D game object that follows the UI hand in real space so that I can interact with 3D objects.

3. Letters that can be picked up if the 3D hand collides.

4. A platform where the letters can be placed after they were picked up with the 3D hand.

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The game is a simple interactive with body tracking. The point of the game is to build a tower with letters on the platform. So you move your hand to a letter and it snaps to your hand then you move your hand to the platform and if it collides with the platform it detaches itself from your hand. This way you can build a tower.

If i move the 3D hand with transform.position every thing works fine except that the velocity that the hand has cant be transferred to the letter because the 3D hand is moved via transom.position.

So i tried to move the 3D hand with rigidbody.velocity. This did change the velocity. But for some odd reason the letter does not want to collide with the platform anymore.

I have tried many things. But I just don't understand that if I move the 3D hand with a certain way 2 other objects can't collide anymore.

EDIT: I used a work around by not parenting the Letter to the 3D object and by just let it follow it by using AddForce. But I still want to know why the collision stopped working whiles I changed the movement of another gameobject.