Make sure when you place the material on the tray that it does not overlap onto the plastic lips at the sides. (It needs to stay over the metal grid portion of the tray or the wheels can hit it and pop the laser arm off the tracks.)
You can gently pop that laser arm back onto the tracks. Make sure you turn off the machine first.
There are instructions to square up the laser arm that explain how to get the laser arm back on the tracks.