The amount of bodies that stay on the field is due to the unit detail setting. Low and Medium = Bodies disappearing, High and Ultra = All bodies stay on the field.
If your graphics are locked, you can always go into the preferences script file located in C:\Users\______ (Your profile)\AppData\Roaming\The Creative Assembly\Shogun2\Scripts
You might also need to set the AppData folder to viewable instead of hidden (I forget the steps, but it can easily be found on the internet).
Hope that helps!