If you mean Pieris "Forest Flame" they need a reasonably sheltered spot,plenty of sun and the most important thing is an acid soil.

Can you grow other acid loving plants such as rhododendron or azaleas? if so you have an acid soil. If not you'll need to grow them in a pot with ericaceous( acid) compost .

Does your garden get salt spray from the sea? If so that may also be causing the plants problems.

sorry Anchorman, a slight correction: Pieris japonica varieties prefer a bit more sun; Pieris floribunda varieties do not appreciate full sun, prefer much more shade, and P. 'Forest Flame' is in the latter group. Otherwise, agree with all your points - particularly the possibility of salt spray, which they don't appreciate.

Point taken. I was not completely sure of my facts on the shade requirements but as I was short of time took a punt without checking !

I agree Pieris Floribunda varieties do prefer some shade although my next door neighbour has a Forest flame in full sun and it seems to be thriving.

A little sideways tip is that you can grow any acid loving plant anywhere that you like. In a container or in open ground. The point is that they find it difficult to get the iron out of the soil, which other plants can readily achieve. So, the trick is to give them loads of iron. No, I don't mean to dump a whole load of scaffolding on top of them, but a nice handful of rusty old nails will be superb, or indeed, anything that rusts.
Having said that, if you are trying to grow them in a limestone soil, use a container with ericaecous compost as correctly advised earlier.