If a travel agent fails to make a reservation for you -- or delays making a reservation for you -- and you lose money because of it, the agent is responsible to you if the failure to make the reservation or the delay was the agent's fault. For example, if the flight you want to take has seats available when you call your agent, but the agent delays making your reservation so that the flight sells out and you have to take a more expensive flight, the agent would be liable to you for the price difference.
When making a reservation, a travel agent must do his or her best to match the reservation to your specific requirements and limitations. If your travel agent makes the wrong reservation and you have a ticket on a plane destined for somewhere you don't want to go, the agent is probably responsible for paying the additional cost of getting you to your proper destination. If the agent books you into the wrong hotel or reserves the wrong type of rental car, the agent should compensate you for the difference between the value you would have received had the agent made the reservation properly and what you did receive as a result of the agent's mistake.