Simple Arm Gripper Not Picking up Objects

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Hey there i am new to gazebo and trying out all tutorials mentioned on Gazebo's wiki page. I am facing problem particularly in picking up a object in Gazebo. the object seems to slip from the gripper's grip . I tried to increase friction on both tip and cube but same thing was happening. I also used <gripper> tag but that also didn't help. if anyone could tell error in my attached code , it would be appreciated. Thank you

<sdf version="1.4"> <model name="simple_gripper"> <link name="riser"> <pose>-0.15 0.0 0.5 0 0 0</pose> <inertial> <pose>0 0 -0.5 0 0 0</pose> <inertia> <ixx>0.01</ixx> <ixy>0</ixy> <ixz>0</ixz> <iyy>0.01</iyy> <iyz>0</iyz> <izz>0.01</izz> </inertia> <mass>10.0</mass> </inertial> <collision name="collision"> <geometry> <box> <size>0.2 0.2 1.0</size> </box> </geometry> </collision> <visual name="visual"> <geometry> <box> <size>0.2 0.2 1.0</size> </box> </geometry> <material> <script>Gazebo/Purple</script> </material> </visual> </link> <link name="palm"> <pose>0.0 0.0 0.05 0 0 0</pose> <inertial> <inertia> <ixx>0.01</ixx> <ixy>0</ixy> <ixz>0</ixz> <iyy>0.01</iyy> <iyz>0</iyz> <izz>0.01</izz> </inertia> <mass>0.5</mass> </inertial> <collision name="collision"> <geometry> <box> <size>0.1 0.2 0.1</size> </box> </geometry> </collision> <visual name="visual"> <geometry> <box> <size>0.1 0.2 0.1</size> </box> </geometry> <material> <script>Gazebo/Red</script> </material> </visual> </link> <link name="left_finger"> <pose>0.1 0.2 0.05 0 0 -0.78539</pose> <inertial> <inertia> <ixx>0.01</ixx> <ixy>0</ixy> <ixz>0</ixz> <iyy>0.01</iyy> <iyz>0</iyz> <izz>0.01</izz> </inertia> <mass>0.1</mass> </inertial> <collision name="collision"> <geometry> <box> <size>0.1 0.3 0.1</size> </box> </geometry> </collision> <visual name="visual"> <geometry> <box> <size>0.1 0.3 0.1</size> </box> </geometry> <material> <script>Gazebo/Blue</script> </material> </visual> </link> <link name="left_finger_tip"> <pose>0.336 0.3 0.05 0 0 1.5707</pose> <inertial> <inertia> <ixx>0.01</ixx> <ixy>0</ixy> <ixz>0</ixz> <iyy>0.01</iyy> <iyz>0</iyz> <izz>0.01</izz> </inertia> <mass>0.1</mass> </inertial> <collision name="collision"> <geometry> <box> <size>0.1 0.2 0.1</size> </box> </geometry> <surface> <friction> <ode> <mu>0.4</mu> <mu2>0.4</mu2> <slip1>0.6</slip1> <slip2>0.6</slip2> </ode> </friction> </surface> </collision>

    <visual name="visual">
        <geometry>
            <box>
                <size>0.1 0.2 0.1</size>
            </box>gedit ~/.gazebo/models/my_gripper/simple_gripper.sdf
        </geometry>
        <material>
            <script>Gazebo/Blue</script>
        </material>
    </visual>
</link>
<link name="right_finger">
    <pose>0.1 -0.2 0.05 0 0 .78539</pose>
    <inertial>
        <inertia>
            <ixx>0.01</ixx>
            <ixy>0</ixy>
            <ixz>0</ixz>
            <iyy>0.01</iyy>
            <iyz>0</iyz>
            <izz>0.01</izz>
        </inertia>
        <mass>0.1</mass>
    </inertial>
    <collision name ...