* Call back to get the truth, not close the sale. Most potential clients who suddenly "disappear" will be expecting you chase them down by calling them and saying, "Hi, I was just wondering where things are at?" Instead, eliminate all sales pressure by telling them that you're okay with their decision not to move forward, based on their not having called you back. In other words, take a step backward. Most of the time, it'll open the door to a new level of open, trusting communication.
* Reassure potential clients that you can handle a "no." Of course we'd rather not hear a "no." But the only way to free yourself and your clients from subtle sales pressures is to let them know that it's not about the sale but about the best choice for them--and if that means no sale, it's okay, because it's ultimately not about you but about them.
* Ask for feedback. Whenever potential clients "disappear," call them back (e-mail them if you have to, but only as a last resort because dialogue is always better) and simply ask, "Would you please share your feedback with me as to how I can improve for next time? Now that our sales process is over, I'm committed to understanding where I went wrong." This is not being feeble or weak -- it's being humble, which often triggers the truth.