That degree if churn is highly indicative of poor employee morale which could be caused by a variety of factors must is most often due to a bad manager or general leadership issues.

Have you tried an anonymous survey or any other means by which to uncover your blind spots?

Happy to talk through the specifics of your situation in a call.

I think you need to provide more information.
ie; industry/space, technology, office location, perks, etc.., avg age of employees, avg age of employees leaving.

I'm in agreement with all the answers so far. I would add that having just read, "Delivering Happiness" by Zappos founder and CEO Tony Hsieh, I'm reminded that having a clear company culture and hiring for culture fit is imperative. As a former HR manger, I saw how inattention to this impacted turnover and organization success over and over. I would be glad to talk over how you have established company values and are realizing those to support the success of your business, and retention.

Leaders all bring different abilities to the table when it comes to managing, motivating, and retaining employees. At your size company it would be normal to have a single HR executive making sure that your company culture is developing in a positive direction, with a shared common energy and goals. Engaging employees in this manner can be difficult especially for full time entrepreneurs who are running all over wearing 50 hats and don't have HR experience. Compliance at that employee count is also a great concern. It is EXTREMELY easy to lose focus of internal relationships as time availability diminishes and the company grows. Many entrepreneurs ego prevents them from fully engaging employees, taking calls, texts, or shooting off quick emails while in meetings instead of focusing on respecting others time where you have demanded it.

Take action now, your losing seasoned knowledge and that can be costly in terms of innovation and problem solving.

Do a company wide survey of all employees and ask 4 things, have them submit the survey printed with no name, to one single person that everyone trusts and make sure it is very clear that it is confidential. Give $200 in cash to the 5 best responses through the mole.
1. What 3 things can senior management do better as a leaders?
2. What 2 things do you like least about working here?
3. What 2 things do you like the most about working here?
4. What off the wall idea would you like to see implemented?

Make sure you and your team read every single response, if you can't find the time to do that then you have already learned you aren't prioritizing the employee relationships.

Institute initiatives to make employees better understand their roles within the organizations, and how their careers can develop. One of the leading reasons of high turnover other than poor communication is a lack of definition of role, responsibilities, and opportunities. People don't like working jobs where they can't see their work develop and mature, or opportunities to advance within the company.

Lastly, always make sure employees are appreciated uniquely and in person. Saying thank you even to the most distant employee to your work flow can be important to that employee in terms of feeling connected to the company. At least annually get the whole staff together and give out awards based on highest performance employees as measured on your employee review scale. Give those employees 2 extra VK days along with the trophy, explain to everyone how they went the extra mile for the company and how much value it provided.

You could also review your employee benefits including time off, flexible work hours/telecommuting, insurances, retirement saving opportunities, etc.