Maybe some people have driven past and your house just hasn't got kerb appeal so they haven't bothered to contact the estate agent.
Why don't you get a really good friend to do an independent assessment on the outside of your house and internally, rather like the House Doctor and give you a honest opinion as to what is wrong. Sometimes we houseowners can't see for ourselves what the problem is as we have lived with it for so long or have very individual taste.
Often the problem is too much clutter, furniture or personal items, that prevent a viewer from seeing how the space should be lived in and how their own furniture would fit.
Anything that makes a house lighter and brighter will help. Also, make it smell nice.
Give the Estate Agent a deadline - such as three weeks - to get some viewers round or change your agent. You could try advertising yourself, or have an open day event, make up your own flyers, lay on some drinks, smarten the house up and through open the doors.