There can be several different reasons which make your range in east inferior than in west:
(1) Maximum range in any direction depends on the altitude of highest flying plane in that direction. It may be possible that flight routes in your area are such that higher planes are in west, and lower in east.
(2) The terrain around your site is such that it blocks view of horizon to east due to hills or land gradient (site located at a point which has much lower ground level than east, and much higher than west).
(3) Trees or houses/buildings in east which have roof line above your antenna.
(4) The antenna is not omni-directional.
Item (3) can be checked by you by climbing up the location of antenna installation and looking in east.
Item (2) can be checked, using method given in my post:
What is the Maximum Range I can Get?
Item (4) does not seem to apply to a professionally designed, built & tested antenna like FlightAware’s. However to make sure, you can use a true omni antenna to see if that gives uniform range in east & west. True omnis are
(1) The “optimized” stock antenna which was supplied with DVB-T.
All these 3 true-omni antennas are given in my post:
Three Easy DIY Antennas for Beginners.