Why does the end effector link of my 6 DoF robot act strange when making contact?

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Hello,

I'm working with a 6 DoF robot in gazebo. I've been having issues with making the end effector behave properly. When I move it through some boxes for example, it seems to get stuck on the boxes as the rest of the robot continues moving past them. Here's the URDF for link_6 and it's joint. I've tried adjusting some of the world physics properties as well. Any ideas that could be causing this?

<link name="link_6">
 <inertial>
  <origin
    xyz="1.51872095159637 -0.579399630103464 1.1458488535294"
    rpy="0 0 0" />
  <mass
    value="0.01" />
  <inertia
    ixx="0.00121172108473992"
    ixy="-0.000289930478786833"
    ixz="0.000101428482694605"
    iyy="0.000887798012340719"
    iyz="-5.46120722442754E-05"
    izz="0.000747466019901337" />
</inertial>
<visual>
  <origin
    xyz="-0.202 0.165 0.115"
    rpy="-1.57 0 0" />
  <geometry>
    <mesh
      filename="package://ms_5_description/meshes/robot_unigripper.STL" />
  </geometry>
</visual>
<collision>
  <origin
    xyz="-0.202 0.165 0.115"
    rpy="-1.57 0 0" />
  <geometry>
    <mesh
      filename="package://ms_5_description/meshes/robot_unigripper.STL" />
  </geometry>
</collision>
</link>
<joint
name="joint_t"
type="revolute">
<origin
  xyz="0.0 0.0 0.0"
  rpy="0.0 0.0 0.0" />
<parent
  link="link_5" />
<child
  link="link_6" />
<axis
  xyz="0.0 1.0 0.0" />
<limit
  lower="-1.57"
  upper="1.57"
  effort="10"
  velocity="5" />
<dynamics damping="1000" friction="1000"/>
</joint>