emily,
all airlines work within a "window" of around336 days ahead of a flight.
So from airlines direct, you won't get a price until 336 days from the date of your return flight.
Comapnies such as Virgin Holidays take your booking but don't effectivly "book" your seat untill the 336 day window, but they can give you a price before that date.
How they work out that price I do not know, unless they have an agreement with the airline??