Die-Back on Conifer Hedge

After a recent cutting, this Leyland Cyprus (Cupressocyparis leylandii) hedge is showing signs of die-back (the brown bits), a problem for a few years now. It hasn't grown much this year; this is the first cut.

However, the other half (about 2/3 actually), which was cut just before the end of the hot dry spell, is clear of die-back.

I gave the complete hedge a couple of foliar feeds of 'Phostrogen Plant Food (14-10-27)' - one in March, then another in April. I think this has hardened the foliage enough (because of the high potassium (K) to nitrogen (N) ratio) to make it less attractive to aphids (which I am told cause the die-back problem, though I have never actually seen any - they seem to have disappeared by the time the damage is noticeable).

To some extent, the treatment has worked. But why on one part but not another is a mystery. There seem to be lots of these, this year.

I have found that if I cut out the dead back to the wood (but careful not to go too far), the hedge grows back into the hole within about 18 months. It would be really good to have a full cure.