There are many reasons why you might be feeling unhappy with your lettings agent, from not managing your property in a way you would expect to locating the wrong kind of tenants. However, there are some various steps that can be taken if you feel as though your agent is not doing all they can for you.
Be direct - The first step would be to discuss any issues or concerns with your agent before taking any action; they should do everything in their power to keep you.
Contracts - If you're thinking of cancelling your agreement, make sure that you read through your contract as it could include a notice period. If your agent has breached their contract, you should not be charged any fees.
Tenants - If you are happy with your current tenants and want to keep them in the property, you need to ensure that your contract does not forbid this. Some letting agents may add a clause that states that you are liable for paying fees if one of the original tenants is still living there.
Final steps - If you have given your letting agent a chance to fix the issue, but feel you are still being treated unfairly, you have the right to make a complaint. Most agents will be registered with a trade association for landlords or letting agents, which you can get in touch with to resolve the dispute.