Accepting your kombo limitations

Anyone have kombos they can't do, or had a hard time learning?

Currently labing Johnny Cage. Have had a lot of fun learning his kombos. I've gotten good at this one: f3,44 bd3 amp >> 121 bf2

I cannot wrap my head around adding f3 after the amped nut punch though I have seen it done well (for example in that Johnny kombo post yesterday). I tried repeatedly and the input timing is so narrow. Anyone encounter something similar?