If your property has been on the market for a while and not sold, the following checklist may help determine what to do next:

1. If there have been no viewings at all, do your own searches online and make sure your property is coming up correctly, e.g. the right type of property in the right town and / or locality.
2. If step 1 looks OK, ask your agent to check how many times your property has appeared in search results (they should be able access this info).
3. Assuming your property has been viewed online by lots of people (it will have, unless there is a technical issue) then something is stopping these people asking to view.
4. Search for your property again, along with others that are similar. Use your postcode and a search radius (e.g. ½ mile) that brings up a few comparable properties, ensuring they are similar to yours e.g. 4 bed detached, and make the price range is quite wide to include properties that are priced lower than yours. Finally make sure you include properties that are Sold STC, very important.
5. Now you can see similar properties that have sold. The sold price will not show but most properties sell close to asking price, so it gives you a good idea. 
6. Start with properties that sold at a price similar to your current asking price. What is better about them? There must be something otherwise you would have sold or had some viewings at least. Is it the size, the location, the presentation, the quality of the photos, etc? Be objective or ask someone who you know will be honest with you (not all estate agents will be, for fear of upsetting you).
7. Now look at properties that are significantly cheaper, is your property more like these? 
8. Unless you have identified something you can do to improve your chances of sale, or something your agent can do that they should have been doing already, then really you are only left with reducing the price.
9. Some people think that instructing another agent will help, that somehow another agent will have different buyers, but of course this is not true. Sure if the general service is poor then change agent but don’t expect different results just because someone else does exactly the same thing.

Nine times out of ten it is simply down to price and this can be hard for vendors to accept. The real value of the property is what buyers are willing to pay for it at the time. Your agent can advise you what new price is likely to make the most difference to results. We also have a post specifically about pricing strategy.