it's about coordination and muscle memory...not just strength.. Scrap's eye trick helps that. Also what Adam said, just use it and use it and use it.

I was in the same position. Absolutely ZERO left hand other than jab. To develop my feel and coordination I did everything very slowly at firsty. It's like learning a new song or scale on guitar or piano with a metreonome and building your speed. Then up the pace....work on power. I've also really stengthened my left hook by doing plyometric med ball work against a brick wall with a 5-7lb rubber med ball. I learned how to hit a speed bag by imagining a metronome in my head and just kept the beat with my hands. Start slowly to learn feel and work your way up.