Why is Camellia sasanqua ‘Setsugekka’ growing so slowly and leggy?

Hi I planted them 3 years ago and it went from knee to waist high now.

I was told it’s the fastest and toughest growing Camellia but it’s growing slowly, leggy and I had to prune off all the sunburnt leaves from summer which made it look even bare.

I’m sure there’s nothing wrong with the soil because all the other plants next to it have already grown fast and are closed to covering the eye sore fence. The garden along the fence lines have been dumped with dynamic lifter every 2 months in the growing season.

Any idea what’s wrong and what I can do to help them to fill out quicker?

Thanks