It is slightly alarming if fertilisation and pruning did not affect your hedge in a positive way – usually the privet is a robust plant which is easy to handle. As I have not seen the hedge it is hard to give any advice. But I will give you some impulses for on-site inspection which might show you what to do. What was the quality of the plant when you bought it? Were the roots strong and sufficiently developed? What was the planting situation like? Plants with open roots have to be transported covered by foil - if being transported on an open trailer - so that they are dried out and damaged by the airflow. Also, the bare roots should not lie in the sun before planting. Did you plant the privet in deep and lose soil? This way it can build strong roots during growth. Did you water sufficiently during growth? How deep did you plant the privet? At best you plant it neither too deep nor too exalted – similar to rose plants. How much did you fertilise? Maybe it was too much in combination with watering and dryness? Right after planting mineral fertilisers should be avoided as the plant needs to enroot first. And never give fertiliser directly to the roots when you are planting. How did you cut back? Strong pruning leads to strong growth and weak pruning to weak growth. Also, if strong plants are right next to the privet hedge, they could be harmed by their shade or root pressure.
But it is really hard to find the reason without having seen the plants. At any rate, I hope that I have pushed you into the right direction with my suggestions and questions.