Hi there.....what sort of compost/earth do you have in the pieris pots? Pieris are of the heath family and so really need acid or neutral soils. They will grow exceptionally well in containers of lime-free, ericaceous compost - somewhat like azaleas and the rhododendrons do too.
They do like a bit of shade - to prevent scorching but frankly - the way you describe your ones going "off" like that and developing yellow leaves makes me think they need better ericaceous compost and a lot more lime-free water. Jolly good luck and I do hope your pieris recovers.

I think the same, rainwater and ericaceous compost sorted mine as did some shade

You might also check the drainage in the pot. Is the compost soaking wet? Most plants hate to be in soil that is waterlogged.

And welcome to GOY, Debbiemay.....hope you enjoy browsing the site and if you get a chance you could write a bit "about me" on your homepage.

You'll soon be addicted to GOY....lots of good advice/encouragement (as you can see from the above helpful comments about your pieris which I hope you can manage to save).

Apologies Debbiemay...I too meant to offer a warm welcome....you will soon find that you are among friends on here. I popped on for advice well over a year ago ...and got hooked ...like so many of us...LOL

Thanks everyone for your welcome and comments.

I have the right soil, so it must be the drainage as had lots of rain the last couple of months and it is soaking wet.I can't remember if I put stones in the bottom of the pot. Any suggestions of what to do next? Think the frost last year also addded to killing off my previous one.

If it is the drainage remove the plant from the pot ,check there are plenty of drainage holes, add some gravel to the base of the pot and stand un an area where it won't get rained on ( roof overhang) until it has time to dry out a bit. Then all you can do is hope it hasn't gone beyond the point of no return

If you have a green house you could perhaps stand it just inside the door but keep the door open as even at this time of year on sunny days the greenhouse will get very warm. Or maybe you could just put it undercover when rain is due until the soil dries out somewhat.

Does sound like you've either got it in a pot with no drainage holes, or have kept it standing in a tray. Make sure it does have drainage holes, and I'd also ask what size pot you have it in - is it large enough? If you repot into something that drains freely, it won't take long at all for the excess water to disappear, it'll go within a day easily.