I placed traffic using the vehicle "create traffic" tool, and when I reopened the file, I got an error stating that, "The scene integrity check has detected some items that could not be loaded. These items have been removed from the scene...contact technical support."
After clicking "OK", the vehicles are gone and have to be replaced.
Which version of MS? you are using to create Traffic tool.
Can you please share the files with us, so that we will try to reproduce the same at our end.