Why do the objects slip when moving them with parallel gripper or allegro ?

[GoktugGuvercin]

I am working on two different robotic arms, parallel gripper and allegro (four fingered humanoid hand). I can easily grasp the objects, and when gravity is activated, the objects do not slip.

After grasping the objects, I want to place them at another location on the table, so I move them there. However, when the robotic arm is moving, the objects start to move inside the gripper or allegro, and slip them. It is like simulation stop performing force on fingers.

How can I solve this problem ?
By the way, I am using position control.