So now you have singled out the clients you believe are not coming back, at least not without effort on your part. How do you reach out to them? You need to contact the people you have built a relationship with and be open with them. Let them know you are concerned that you haven’t been their supplier lately and want to know why. If they hedge, don’t leave it. Let them know that you are really invested in keeping clients and if you need to change some of the ways you do business to achieve that, you need to know where you went wrong.
Admitting to a client that you may have made a mistake is not a sign of weakness. It is a sign that you are based in reality. You know that sometimes mistakes are made that cost you clients. If a past client opens up, you have a chance to make amends. Ask them to be part of the process to improve your service. They may or may not be interested in that, but it never hurts to ask.
That past client may still not return but the openness they provide goes a long way to making sure it doesn’t happen with others. However, the door may not be shut permanently. Given time, that past client may be a returning client once you prove you will do what you need to in order to recreate that relationship that brought them to you in the first place.
Building professional relationships is always work; don’t imagine that it isn’t. Just remember the rewards that come with the work.