Hi Andrew,
I have some varieties that have not produced in over 4 years. A fig tree when very young (less than five years in my book) is unpredictable. Some varieties will produce almost immediately, while some others, might take much longer to produce. Then there are some varieties that might not be too happy in a pot. I have many of those shrubs that are over four years old and still waiting for some fruit. I don't really know if there's a formula. As an example, you could have a Celeste, but not all are equal. I had two strains of Celeste, one never let me down since I got it, while the other one kept dropping its fruit. I eventually let it go.
I have two strains of Col de Dame. The UCD strain despite some claims that all UCD figs have FMV, is a wonderful producer while the other one whichl was supposed to be FMV free has'nt produced in over four years. I have no answer for this.
I you have a proven producer in pot, patience is perhaps your best bet. This year I was surprised by one of my trees which I was about to discard. After four years it finally produced. So continue to feed your trees, and I'm sure that eventually they will surprise you.