A late frost can do that to a piers... I'd wait and see if they recover.

You don't say how long you've had the plants, but I suspect not long, and that the problem is your sandy soil. Pieris needs a humus-rich soil, so I would advise you to incorporate lots of organic matter - leaf mould, garden compost, etc.

Can you remember precisely which varieties they are? I ask because Pieris forrestii varieties like a lot of shade, whereas P. japonica don't mind sun... and how long have you had them in the ground?

Hi thanks. They have been in the ground for about 5 years or so. They are Japonica. We have added a fair amount of ericaeceous compost over the years. We have not had any serious frosts recently..Thanks alot

In the background on the left hand side, I can see what looks like another Pieris - if it is, that one looks healthy, unlike the ones in the foreground. Because your photo is a closeup, I can't tell whether the ones in the foreground are more exposed than the one behind, because the area around the plants is not visible, but if they are more exposed, they may well have suffered frost attack whether you were aware of a hard frost or not - or, the other possibility is, have you used any products which kill weeds nearby, via sprayer or watering can, or applied any lawn treatments near these shrubs?

If neither of these things fits the bill, check the woody parts of the plants, particularly towards the base, and the backs of leaves. You're looking for anything that shouldn't be there - insect or scale infestation, soft or weeping areas on the woody parts, black rotting areas near the base, anything unusual.

Have they only recently gone like this, or have they struggled since they were planted? the leaves look mottled and yellowing, have you been over feeding them?, check for pest and disease like bamboo says, sometimes scale can be hard to see so if you get a magnifying glass, then check closely on the stems, if you can't find the cause then if it were me I would lift them and move them or pot them up into big pots and bring them back to vigour by feeding correctly and putting them in the in the correct position.

Ok thanks i will do thorough check for pests. With regard to feeding, i gave them 2-3 feeds of ericaceous liquid feed last year and one this year. The soil is not acidic and probably bad idea to plant them in first place-stupid!

Not stupid, just means you have to work harder to keep them happy :)