There are numerous reasons why some listings don't sell. I've compiled a list that may help answer this question.

1.The property is priced incorrectly, usually, much higher than market value.
2.The "value" to a potential buyer is NOT present
3.There are more sellers than there are buyers... in a market with more than 6 months inventory can be considered as a "Buyers' Market"
4.Insufficient or lack of quality photos of the listing. I can't tell you the numerous times I see photos of the toilet seat up in the bathroom...
5.The yard looks like a dump
6.Sellers' not willing to allow showings
7.The listing agent is difficult to work with
8.The listing isn't staged or in showing condition... The key to best present the home to potential buyers is to DE-PERSONALIZE. Remove personal pictures, photos etc to allow buyers to imagine THEMSELVES living in the space.
9.The "For Sale" sign is inadequate or is not present
10.The property isn't marketed well by the listing company