One of the key retention challenges is meeting people's career expectations. Inevitably, many people want to move upwards quickly; sometimes this just isn't possible.
What you need to do is discuss expectations from the very beginning. Some seemingly ideal candidates don't seem quite so attractive once you realise that they expect to be promoted almost straight away, otherwise they'll be off, causing more disruption, more trouble than it's worth. Other candidates, perhaps slightly less driven, less ambitious, may be a better fit for your situation, their aspirations in line with your company's typical career progression timeframes.
Current employees may also have unrealistic expectations. In this case, their manager or supervisor needs to use a bit of imagination, creativity, flexibility, to see if a compromise can be reached. Perhaps providing some developmental coaching so the employee can gain the skills they need to gain a promotion.
It is no use simply telling employees that their expectations are unrealistic, that almost inevitably sends them towards the exit door. There needs to be a sense of progress, a clear pathway towards their medium term and longer term career goals. There needs to be a reason to stay around.