a short while ago put some Pieris Forest Flame in some pots with some
new john innes no 3 soil.

You need ericaceous compost. If you can't easily repot, add plenty of eri
fertiliser.

FWIW pieris struggle in our neutral garden soil. They live, but don't
thrive. However, the ones in the pots with the right compost have reached 5
feet tall and nearly as wide. They always have a great show of coloured
leaves and flower profuciously. The bees love the flowers too

Al.