I'm assuming it's getting good sun. Otherwise, I suspect that there is some problem at the roots (maybe involving the soil). How's the drainage? I'd be tempted to pot it up in some decent soil and just grow it containerized. It should be growing like a weed at this point.

They need lots of water and sun and good soil. Miss one of three in the right amount and it will not grow. I've got one growing in a pot on my patio. I wintered it over in the house pot and trellis. It's filled in from thin look of winter and has lots of flowers.

Mine is in pot. Plenty of sun,water an