GDS availability
The problem with all GDS systems, as far as I am aware, is that they can only show availabilty up to somewhere in the region of 330 days ahead. I am sure that LAX_LHR knows more about the actual timescale than me. Additionally all airlines that I know of only have one specific season on sale at a time (with definite schedule), so Summer 2015 is only fully loaded once Summer 2014 is closed.
I accept that my information may be out of date and look forward to being corrected if necessary.