Did you experience this as a buyer? Or, as a seller, is it only your listing agent who has brought buyers and you are assuming other agents aren't showing your property in favor of thier own listings?
If this was your experience as a buyer, I am sorry to hear that and can assure you that is rare. Most buyer agentsare completely and wholeheartedly dedicated to helping their buyers find that best property for them. Aside from legal requirements of fiduciary, the bottom line is that most buyer agents are motivated by a sincere desire to achieve the goal of finding you the best property for you. Realtors derive great satisfaction in knowing that they played a role in your happiness with your new home. If your buyer agent is not sharing your passion, it wouldn't be hard to find one who will. Giving benefit of the doubt to the current buyer agent, I would suggest that you voice your concerns on thie matter with that agent first before jumping ship to the next agent. Perhaps his/her detailed knowledge of his/her own listings casued more focus on those properties and accidently painted a picture different than intented.
As a seller, if only your listing agent is showing your property, inquire about broker marketing, ck to see if the buyer agent compensation is competitive, and ck your pricing to ensure it is competitive. If your property is widely exposed to the broker community, and properly priced, you will get showings from other agents.