First of all, change how you end the sales call. Before you leave set up the next step. Ask the prospect how they'd like to proceed. What do they think so far? What are their next steps? What is the next step for the two of you? Too many salespeople get caught up in how well the meeting is going and convince themselves they've got the deal. They walk out all smiles - and without a plan. Make sure you define with the prospect how you will move forward.
If the next step was for you to call and follow up, and you've done that with no response, it's probably time to leave the pleasantly persistent message. This is where you let them know you are starting to feel like a stalker. Without any emotion, let them know that while you are very interested in working with them you don't want to feel like you are pestering them. You appreciate how busy they are and understand that something may have happened to derail the sale. Tell them the action you are going to take unless you hear back from them otherwise. For example, tell them you'll be in the area and will stop by to see them next Tuesday unless you hear back from them before then. Let them know that if things have changed it's okay. Sometimes just giving them permission to tell you 'no' is enough to get them to respond.
Things can change for a company and they may not be able to move forward. They may not be able to move forward with you. If this makes them feel uncomfortable, or they think you aren't going to accept 'no' for an answer, they are going to avoid you. However, their silence may just mean that they are really busy and that this initiative has taken a back seat to other, more important things. They may not have been able to get to an answer just yet due to the busyness of other people within their company. So many possible reasons. Don't assume. Do reach out in a way that creates a response.