I have a lot of lilly pilly hedges (Acmena species) and the ones growing facing north are doing well and the ones growing facing south have not bushed as much as I would have liked, but I bought some Confidor tablets (a packet of 8 ball shapes) and I put the handle of a tablespoon into the soil (to make a deepish hole) on one side of the Lilly Pilly and put one Confidor ball into the hole (made by the handle of the tablespoon). I then did the same on the other side of the Lilly Pilly. Ideally, before planting your lilly pillies you should put one Confidor ball tablet into the centre of the hole dug to put your lilly pilly into.
The confidor balls are fantastic (especially for Syzygium smithii) as they fix any physillids and they also feed and nourish the roots of the lilly pilly hedge. After putting the Confidor ball tablets into the soil, I then watered the lilly pilly hedges and I did notice a difference in a short space of time. I usually repeat putting the Confidor ball tablets every 3 or 4 months just for nourishment and protection.
I had one lilly pilly (facing south) that got burnt due to extreme hot weather and I had to cut a lot off the top and it took a long time to catch up to the other hedges next to it, but I persevered and even though it looked uneven and silly, I just kept watering it and eventually when I cut the tops of all the hedges, I noticed that the burnt one had caught up and now it is looking so much better and they are all nice and high.
I also spray my hedges with Confidor pest spray (whenever I remember) and now I have lots of white butterflies flying around.
Don't be afraid to cut the tops and the faces of the lilly pilly hedges because it does help them to grow.
I hope this has helped you in a small way.