There's nothing more depressing than getting feedback on one of your own and hearing that he /she made not a great impression, not a good impression, but was unimpressive. Depression over an impression.
How does this happen? Failure to meet expectations? Sitting like a bump on a log? Failure to reach out and grab the prospect's attention? Too much talking? Boring too? Lack of good, tough, timely questions? Not breaking new ground? Not likeable? Offensive? Doesn't present him/herself well? Ineffective opening remarks? Vague positioning statement?
Your first step is to determine whether you want this salesperson to represent you any longer. If you do, identify which of these possible issues are the problems. Then, you must present the issue to the salesperson, get acknowledgement of the problem and commitment to change. Finally, it's coaching, coaching, coaching until the problem has been rectified.