I have sunshine ligustrum and since I have no idea what your soil is like, I'll tell you what I think might be wrong. When you planted did you loosen the soil a foot down and wide, add compost and mix that in with your soil?
Plants like a good drenching weekly not drip drip drip. They need to have water deep down in the soil so they will grow roots deep down. if the soil stays moist only in the top layer, the roots will stay there, not grow much because the soil beneath is dry so, no point in going there with their roots. Lack of nutrition and water accessible in the subsoil = plant that cannot grow, they can only survive.
I would dig it up and put it in a bucket, spray the rootball with water to keep it moist and remove most of the old soil .Now dig it a new hole, get rid of a couple shovelfulls of the old soil, add compost, mix it in the soil that's left. Now fill the hole to the top with water and when that goes all the way down, replant and water again. Get rid of the drip irrigation. It only keeps the upper soil constantly wet. Plants need to have the upper inch or so of soil dry out and then get watered again deeply when it rains. You know, like nature does.
I'm not saying that drip irrigation doesn't have it's place. For vegetables that need consistent water like cuces, watermelon, plants that need constant moisture by nature, it's fine for, but plants that want and need to get their roots down into the soil, they need deep watering once a week, so it isn't good for them IMHO.
