Your question is full of hypothetical possibilities, any of which are quite possibly wrong. Before you immediately conclude that your customer's assistant is the culprit, there are some other places to look.
In the first place, the reason you are not hearing back from this customer may have nothing to do with his assistant. Perhaps your customer is busy, uninterested in the articles, or believes you are not looking for thank-you notes. He just might believe that part of your job as a professional salesperson is to provide these types of articles, and the real sign of his responsiveness is a sales order.
You may also want to think twice about putting labels on his assistant. If she is truly acting as an aggressive gatekeeper, she is probably doing so at his behest. In addition, if you expect to have problems with her, you greatly increase the likelihood of just such an outcome.
One way to get an answer to all of this is to meet face-to-face with this customer more frequently. That is still the best way to build trust, relationships, and sales. More "face time" can also assist you in dealing with his assistant.
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