It's no big secret the BA are very stingy when it comes to handing out upgrades. As it says in the FT upgrade guide, BA like to keep their premium cabins premium for those that have paid for tickets or used miles to upgrade.
It also depends which routes you are flying, as some F cabins are regularly filled by fare-paying passengers (eg. JFK). Also, on relativley empty J/W/Y routes you will not be upgraded to F because the seats in J are not needed due to no overbooking in W/Y.
Perhaps you could use some of your hard-earned miles to upgrade yourself to First next time?