Certainly try some of the suggestions above first, but I have a few observations. I have seen instances where one can only shift with a direct pull when there is in fact a problem with setup. What is sometimes overlooked is that a direct pull can move the derailleur much faster than the lever does. The result is that the extra momentum of the chain can overcome other problems.
The fact that it has been a problem since new means that either the derailleur/shifter is defective or some other aspect of the original setup is not correct. For the defect option I would suggest substitution install a different derailleur and see if it shifts correctly. Trying a different crankset would be a bit more complicated - I'm not familiar with FSA, so won't venture an opinion regarding whether they have general problem.
As for setup, the OP has apparently eliminated outer limit (I assume if adjusted for consistent upshifting it either overshifts or causes constant rubbing on the middle chainwheel) derailleur height/rotation and cable tension. I'm not aware of anything in the shifter that would cause reduced cable pull, so I would eliminate the shifter. That leaves the cable/housing. I would suggest at least checking the housing sections where they seat into the housing stops to make sure there is not a problem causing the housing to give too much on being stressed for the shift to the big ring.