The tips and edges of the leaves are turning brown - why?
These are classic symptoms of over-feeding. Water well to flush through excess fertiliser. If all the leaves drop do not throw the plant out straightaway. It may recover and produce new shoots once the chemical balance between root and soil has been restored.

Remember - only feed between late March and August
I found this on the internet for you, it may help.

OR - it could be chlorotic and need a dose of sequestrene! If it's not in acid soil or compost, that could well be what's happening. If the discolouring is some leaves and they're completely yellow-y, that's likely. Mine is in a pot of ericaeous compost, but I have to water on sequestrene every month or so, when I see the yellowing.