Gazebo allows one model to be inserted and then it freezes

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Hello, I am trying to insert a custom model into gazebo but most of the time upon inserting the first model it sits on the floor and then when inserting another of the same model nothing happens. If I select the model through the 'world' tab then gazebo freezes completely. Occasionally the object will also fall through the floor. The same thing happens on multiple machines. Any idea what the problem could be?