It's a function of outside air temperature. Essentially the condensation you see behind airplanes is air that is warmed up from going through an engine and cooled down afterwards often forming ice crystals. When you see several aircraft, some leaving a trace and others not, it should be due to their different altitude. You can have layered differences in air masses, sometimes within a few hundred feet. Also jet streams can be very localised vertically and will influence the condensation of water vapour.