Frustrations mount when something that is supposed to be easy, isn't. Survey your clients at the end of large matters and always at the end of a year. If you don't want to retain your clients, however, you may go on to the next item. And, if you have no intention of addressing any problems that arise, do not survey them in the first place. Personal interviews with your clients are even better.
Objectivity is key.
Train your assistant how to address your clients and handle calls and important matters in your absence. To some extent, the formality of your firm will dictate this, but certain clients prefer certain forms of address - you should know what they are. If you don't, ask. The last thing you want to do is offend a client with something that has such an easy remedy. And, never blame your staff - it's your responsibility.