You can keep your partners on their toes by occasionally reminding them that they do have competition. If you feel as though you're getting fleeced, call a competitor and get a price quote, then show the quote to your sales rep.
Bad feelings can also develop out of the sheer necessity to go with a particular vendor.
In some cases, one vendor may have a lock on a particular vertical part of the market. This is partly because of the company's long-time presence in the market and partly because the vendor makes a flexible product that can be deployed in a large number of scenarios. If you don't like the vendor's pricing, policies, service or support, you're pretty much stuck. You may be able to find another network analyzer that fills the gap, but chances are you'll need two or three products to fulfill the role that the one product played in the past. Furthermore, your staff will have to be thoroughly trained in a new product.