Because of the late onset of Autumn weather, we've been able to continue planting hardy plants in what we call 'fill-in' spaces -- spaces where plants haven't made it through the season for one reason or another.
No matter how good the weather, or how good a gardner, sometimes some plants just don't make it. Sometimes they've been in a while and have died naturally. Other times, it's something else. Fluke positioning or micro-climate issues in a garden can mean that some plants thrive and others nearby don't. We see this in almost every garden to some extent. A plant in more sun gets larger in time while the same plant two or three down the line struggles in part shade.
At Hedgehoggs Gardening, we try to work with the natural environment as much as possible to ensure our client's garden look great and we aren't having to replace plants too often. At the end of the day, a dry loving plant isn't going to survive in a wet spot just as a sun loving plant will balk if it gets too much shade.
No matter how wonderful a design drawing looks on paper, if the plants don't like their position when they're planted, it's best to find an alternative spot for them while you have the chance and try a plant that is better suited to that particular spot.