Q.My privet hedge is really sparse at the bottom- what should I do?
The privet was planted approx 5 years ago. It has never really bulked out at the bottom half and looks really sparse other than at the top. For the past two years I have tried constantly trimming the top to encourage growth at the bottom, and this year I also thinned it out down the sides to ensure light could reach the inner part but it doesn’t seem to be stimulating much new growth. I have fertilised it every year and planted with bone meal.

Certified GKH Gardening Expert
Yearly fertilization isn't completely necessary, as they are very low feeders. Too much nutrients will cause a lack of growth too, and sometimes leaf burning and brown spots.
If this is the side that doesn't receive much sun, then this will be the reason. When this is the case, you will need to do some severe restorative pruning. This requires cutting almost completely to the ground. This will help to restore a sparse shrub, and make it more full again.
This article will help you to grow these shrubs: https://www.gardeningknowhow.com/ornamental/shrubs/ligustrum/pruning-privet-hedges.htm