This occurs because some groups are falling down, far away from the camera. If you unhide all groups and layers, and then play simulation, you will notice some objects falling infinitely down. You may set their state to ignore or static, so they don't interact with the simulation. Then you can hide them back if desired. If you want them movable, then simply add a huge static ground below so that the falling objects don't fall down beyond it. Also enabling Continuous Collision mode for all objects in Simulation tab may help in preventing objects from falling through each other in case they do so.
