I am by no measure an expert on this, however, when I have headspace issues after racking I flow CO2 into the carboy and then install the airlock.
If you've got CO2 on hand, bonus, it's another highly recommended method on the list of things you can do about headspace, but we don't always mention it to new-bees because not everyone has a CO2 tank kicking around to flush out their headspace.