robot_state_publisher posting wrong tf tree (missing connections)

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Dear ROS community,

I have the following problem:

I'm trying to simulate a robotic leg using ROS kinetic and Gazebo.

In order to get information about the current frame of each link I'm using the robot_state_publisher. He's a subscriber of the /joint_states topic and also receives information from the parameter robot_description which is loaded as an parameter using the xacro file, which describes the leg construction with all it's links and joints. He then publishes to the topic /tf

When I use the rqt_tf_tree to check the tf_tree I have 3 unconnected tree structures as a result, two Joints are missing. I first thought the xacro files would be incompleted, but I checked the robot_description by using $ rosparam get /robot_description

E.g.

<joint name="joint_B1" type=" continuous">
  <origin rpy="0 0.512690467 0" xyz="-0.05 -0.005 0"/>
  <parent link="upper_leg"/>
  <child link="connector_2"/>
  <axis xyz="0 1 0"/>
  <dynamics friction="0"/>
  <limit effort="10" velocity="10"/>
</joint>

is one of the missing connections.

Do you guys have any suggestion where my problem could be?