United Kingdom 
Hi i bought two cotoneaster x wateris a couple of weeks back and have planted each in a large container. One has lovely evergreen leaves but has suddenly started losing them all and the others leaves started to turn light green/yellow almost immediately but has retained them all. They are both watered daily and the containers drain well. They are in good light outside but i wonder if they need to be well sheltered. Would very much appreciate advice!
3 Aug, 2017
It would be very useful to see a photo of each, to be honest, otherwise any answer is speculative. These plants like full sun or part shade, so if one gets more sun than the other, that might make a difference. If the containers are much, much larger than the original rootballs, that can cause a problem. Watering well and regularly is essential for new plants, and there should be good drainage from the pot, in other words, the water is not retained in any outer tray or container after watering, and the drainage holes from the pot are not blocked. They don't particularly need a sheltered spot, unless the weather is extremely cold and they are fully exposed to harsh winter winds, when they might suffer some damage. I'm assuming you used freshly bought potting soil to plant them into.
Otherwise, check backs of leaves and stems for signs of disease or infestation, things like aphids, signs of webbing (cotoneaster webber moth is a possibility if there is webbing), scale insect.
3 Aug, 2017