conifers

Very often it's a question of poor light - they don't like shade or being next to other plants. They love their splendid isolation.
But have you checked for spider mite? Just reach in and check what's there. Beware conifers can irritate! It's part of their defence. Use gloves.

Epsom salts provide the trace elements they are short of. Bags can be bought at garden centres reasonably cheaply.

Mine have too, just arount the bottom section - I caught my cat peeing up them the other day so I think this has harmed the trees. I had to dig them out as they were so badly damaged.