pick an object using spatial contact force

hello, i am implementing pick and place for 3prr delta robot on simulink, like 3rrr delta robot on library made. i put spatial contact force between end effector and red part and points on each of them, when i give it - 20 force for picking up part when end effector approaches it, the part bounces up and down the end effector. i donot understand what 's wrong or why it bounces up and down.Any help, i would be thankful.