Most agencies do not require long-term contracts or demand exclusivity but your contract should state this explicitly so there is no confusion. You should have the ability to cancel your contract at any time for non-performance.
If you believe your selected agency is underperforming, send a batch of accounts to a different agency to see how they compare. Competition is good for you as the client. It is vital that you do not send the same account to more than one agency as this is a violation of the Fair Debt Collections Practices Act (FDCPA).