Hi maxi,
I don't know the Smith fig but :
Can be that particular spot : Do they get sun there ? Do you water regularly?
If so, then an hand full of fertilizer (10/10/10) every two weeks will correct that next year. Here I started fertilizing in March for the roots to make reserves before the buds do break.
I never have fears to fertilize figtrees ... The only problem is with the seedlings ... They are weaker and burn much more easily ... But let's blame the slugs for loosing some seedlings .
My "Longue d'aout" has been a slow grower for me . I'm fed up with lda. So this year I doubled the fertilizer dose ... and it grew already better and surprisingly thickened a bit, but still not as fast as my other trees...
Could be the spot where I planted my lda, as some other members on the forum report good growth on lda ...
I'm now keeping up with that fertilizer program, and if the lda tree burns, hey well, I will have a new free spot for another more productive tree !
And if she succeeds, guess, I'll get more figs from her next year !