Staff in a busy office can sometimes have dozens (or hundreds) of emails waiting to be answered and might take several hours (or several days to get around to even reading them, let alone responding to them).
By the time that your email gets read, the job will probably already have been given to someone who PHONED the agency in response to the same email that you got, meaning that there's little point in replying to your email. (It's far harder to ignore a phone call than it is an email!).
STOP emailing back to express your interest (and/or to find out what the details of the job are) and always PHONE the agency to do so. You then stand a FAR better chance of getting the job.