You call them back a few times and their motivation to buy or sell has not increased. We have all spent large amounts of money on mailings to people who are not moving. We have also spent large amounts of time following up on poor leads. Stop working with people who are not committed to doing something now. The old belief of great salespeople should be able to sell ice to an Eskimo is false.
Great salespeople have clear definitions of what constitutes a lead.
They clearly follow their narrow definitions. They prospect enough to keep a constant supply of leads flowing, so they can qualify and define them carefully. As it says in the Bible, Jesus separated the wheat from the chaff, gathering his wheat in to the barn and burning up the chaff. We must separate the good kernels of wheat from the chaff of unmotivated sellers and buyers. The kernels of wheat have value, the chaff has no value.