Where are you? My full sun might be different than yours. In Sacramento, I can't grow michelia figo in full sun, or even close. But I think that maybe for you, the problem might be too much water or maybe over fertilization. They like regular water but like to dry out a bit between waterings. Michelia figo is not the fastest growing of bushes, no matter how much you fertilize. That's a small guy and it will take a few years to reach even medium size.