Hi Buddy- I just saw your question (a little bit out of the area I work). Have you had an analysis done of what your house should rent for? I am near Baltimore, and Craigslist is by far the primary source for rentals. You should contact an agent and get an estimate of the fair rent value.... you may just be asking too much rent.
Are you close to any of the big employers like PAX Naval Air Station etc? Maybe you could put up some flyers....... also, try police stations, barracks, schools and hospitals.... always a good source of renters with stable incomes. Good luck!