Our policy is to answer emails within 24 hours, except on weekends; associates who do not comply are reprimanded and, if recurrent, fired.
With all due respect I think you should start overseeing compliance with this policy more, because it is a recurrent complaint from many of your firm's clients, and usually some form of improvement in communications between a particular attorney and a client happens only after contacting you (in a way you indicated above). But I do not think that it reflect positively on your staff (and your firm) if the only way to get things resolved is by calling the Boss (you).