Robots change shape and are fixed/stuck.

Hey, I've been using 25 robots of the same type in a swarm simulation in **Gazebo 2.2.3** on **ROS Indigo**. After about an hour of simulation at least half the robots are fixed in the centre of the world and I cannot move them with the hand tool. They do not respond to their subscribed topics, and actually change shape. Using the service: '/gazebo/reset_world' does not reset these stuck robots. In the attached picture ( http://imgur.com/a/YKoYN ) are some of the normal robots and in the bottom right are about 10 fixed/stuck in the same place. I can't find anything like this online and there are no errors printed on console. ---------- The robots themselves use the **differential_drive_controller** plugin and are essentially the robots described in this tutorial: http://gazebosim.org/tutorials?tut=build_robot. I have no idea how to address this and would much appreciate any ideas.