A customer is always too busy to see
you, especially if you have made an appointment and driven
a hundred miles to get there at 8.30am. When you next visit
with your boss, the same customer will casually remark,
"We've not seen you for a while." 
However, if they need something, usually because they forgot to place the order, the customer will expect you to drop everything and alter the course of the universe to get it to them. The amount of times this happens per customer is inversely proportional to the value of the business you get from them.
You are certain to find customers who just won't buy from you for no logical reason at all. You've shown them the product, got clinical approval, and got the price right, but still you don't get the order. At this point you ask yourself if you've got a body odour problem. You haven't? There must be another reason then.
Maybe the product is the wrong colour? This can be very important for a piece of highly specialised medical kit that spends the majority of its working life out of sight within a body cavity.
The colour's OK? Then choose another battle to fight. That customer is probably working to a hidden agenda involving large amounts of sponsorship from the opposition to allow him or her to attend clinical meetings in Hawaii. (Golf optional, naturally).
Or, it may just be that the customer really doesn't need your product. His clinical or intellectual needs are being met elsewhere. Don't tell your product managers, but products even better than yours may be out there too! Your customer may even dislike you and doesn't want to buy from you. That's life, I'm afraid.
Never lose sight of the fact that you need customers to pay your mortgage. Even if they don't buy from you this time, things may change in future. For instance, most customers have a 'Pet Rep'. If he or she leaves the area and you've done the ground work, you may be looked upon favourably next time the contract comes up for renewal. My first boss told me, "People buy people" and that's proved to be the best bit of advice I've ever been given.
The beauty of this job is that the longer you do it, the distinction between 'customer' and 'friend' becomes less clear. I've made a lot of genuine friends in the years I've been a rep, which is exactly how it should be.
N.B. This site looks best when viewed from a distance after drinking six pints of lager.