I have a good idea. The plane load should start at 0. When you increase it, it takes time to load fuel and passengers and cargo. You have to wait at the gate not moving, until the time is passed everything you want to load is loaded. Similar to starting engine, you have to wait a while. But there will be a button that say if you want to load immediately, similar to starting engine.
Similar to this idea. You should see people moving in the cabin when you have loaded people on the plane.