Well, I also had a AML/ST who was scoring on most FKs without having the SA. And even though I kept his shooting attribute reasonably high (say, above 120%), his performance in FKs plummeted after a while.

I speculated that it was because his Strength attribute had gotten too low (Strength is trained by the drill "set pieces delivery", interestingly). But even though I trained strength again (specifically with this drill), he never got back scoring on FKs.
Who knows if it is not in fact another expression of the rumored "hidden attribute"?