Before we all give you advise about what you could do, you have to see if you signed anything that obligates you to the seller's broker for a commission. If you signed a Buyer's Representative Agreement with the agent that you had present the offer, YOU may be obligated to pay that broker a commission if you buy ANYTHING during the time period of that Buyer's Rep agreement. There is a difference between a contractural agreement between a buyer and a Buyer's Agentagreement and Agency Disclosure. You need to review what you signed in addition to the purchase contract. If you did sign a Buyer Agency Contract and you feel that they have not lived up to the terms of the agreement, call that agent's broker request that you be released from that agreement. If you have not signed an agreement, check with your local/State Real Estate Board or agency that governs licensed Real Estate Brokers. In most situations, Procuring Cause dictates if a broker is entitled to a commission or not (that means you got the deal).
Dual Agency has it's pitfalls especially if that agent has the house you like listed. With informed concent, you can work through the listing agent understanding thathe/she is a Seller's Agent.
You really need to see what documents you have signed first. You should also go on line and find a copy of the Agency Disclosure for your state and review it thoroughly first.

Have you thought about doing a dual agency ? That means you work directly with the listing agent to negotiate an offer, and that listing agent still works for the seller. The listing agent represents both parties to the transaction and most likely the listing agent won't have to split a commission. The agent keeps more money in their pocket, and that agent might do more to get the offer accepted.

A separate signed dual agency agreement has to be signed by both buyer and seller. I have done this in the past with some bank owned properties.

Latest Deal:
Only 6 Days on Market with multiple offers
List Price $49,900
My Final Price: $47,500
House is worth after $15-20k repairs about $110,000

What type of an agreement did you sign?

the simple answer to your question, T, is yes! If you have a lack of confidence in your first agent, you are entitled to use whomever you choose.

It's possible that agent #1 might still be due a "finder's fee" from agent #2, but that's internal stuff, that you don't have to worry about. If you feel that you can more effectively negotiate for the same house with a different agent, that's your right, and you're allowed to do so.

But don't just assume that the seller is going to negotiate any differently just because you're coming to him with a different agent. Sometimes sellers can be stubborn (just like buyers), and you may find yourself in the exact same position.

I would certainly have conversation with the agent and if no satisfaction, contact the office manager of that brokerage. Be sure you fully understand all that has transpired and what the agent has done (or not done) to assist you. The manager may be able to move you to another agent in the office if you are not comfortable with the agent you have signed the contract with.

If the agent is working for you, he/she should be explaining the full buying process and what you may need to get your offer accepted.

T, an agent does not have control on whether a seller will accept or reject your offer. There could be several reasons your offer was rejected. Perhaps your offer price was to low, or maybe you are not putting a large security deposit down. Or maybe you were asking too much from the seller. It's hard to say. But if you do not feel comfortable with your agent, and you have not signed a broker/buyer agreement, you are free to work with any agent you choose. Good luck.