You don't want to or cannot contribute more, then think about taking greater investment risks. Of course I do not suggest doing anything you are not comfortable with and that you haven't discussed with your financial advisor. However, as we get closer to retirement our portfolios begin to become more conservative. This is done so an investor will not lose large amounts of retirement money in the event the market fluctuations eat up large parts of your capital (capital preservation). More aggressive vehicles come with a greater potential for growth (and potential for loss). An investor does not have to place all of his or her money into a more aggressive vehicle but can allocate a portion of their money to it.
If taking a riskier position doesn't feel good to you there is another option; increasing your income by creating more streams of revenue:
1. Write a book about the field of your expertise
2. Begin to offer webinars and trainings on your favorite hobby
3. Start a business you've always dreamed of to create more revenue to put away.
The cost of entry into many business opportunities is very, very low these days. Take advantage of it.
These are three options to look into. There of course are more but I hope these got your brain going and hopefully this inspires you to be proactive. Do not avoid this inspection of your money, doing so could mean prolonging the years you get to retire and it could mean less wealth generated for you and your family.