Is this in a container or the ground?

The question above and how recently was it planted? any chance of a photograph?

When you have planted in new compost there is no need to feed for at least six weeks normally. Sequestrered iron shouldn't be needed in ericaceous compost.

Why are you feeding it? Newly-planted shrubs don't need feeding - you should just let them get on with finding their feet and settling in. If you've planted them in the right soil and aspect, they should grow happily without any further help.

Vine weevils like Kalmia (especially when planted in containers but also when in the ground), but as you've only recently planted yours, and I imagine you checked the rootball for vine weevil grubs and other nasties before planting, I would suggest that the problem is overfeeding, but see what others say.

Depends what Foxhaven means by 'sequestered feed' rosierose - he/she may just mean Sequestrene iron tonic, which isn't a fertilizer or feed at all. But its use in what might be a short space of time (what does recent mean, exactly? six months? one month? a year?) is why I asked precisely how long the Kalmia's been planted...