A perfectly hardy plant may have been raised in a polythene tunnel or greenhouse to bring it on quicker. It will be like a London bond dealer expected to perform to the same standard in a new job as a horny-handed son of toil. He will probably eventually get there but not before suffering a lot first. If they aren't happy, then it's because one or more of these are wrong. Think about what it might be - look at it from the plants point of view.
Tips Do some local research , look around at what is in neighbours gardens.
You can't domesticate plants , if nature has decreed they will grow to such a size, then they will, they won't know that you don't want them to. Plants that naturally grow on acidic soils won't survive on chalk and won't do well in neutral soil either . They may spend years hanging on hinting that they might just repeat that initial glory next year, but never actually do - Azaleas are particularly good at this. Keep an eye on your new acquisitions when they are first planted out . Coddle them a bit.