Retaining Good Employees Question:

You need to meet one on one, in an unstructured way, with all your best people at least once a quarter. Quietly and ask them what is frustrating them about their job. What they want to be doing but are not getting to do. Be friendly but blunt. You need to learn. Get it out of them.
You may think if you have drinks together or socialize together that you will know if they are happy. But you will not. Even if people complain in those contexts, it will be general complaints. You will not learn or know, what your top people need to find their growth path at your company. Where they feel stalled out and frustrated. You have to ask.