If you’re running a business, eventually you’ll run into a problem client. For whatever reason, there will always be people who decide to stop paying, stop communicating, and just seem to disappear! You cannot let these people go- but be professional!
You have to be persistent with problem clients. Be committed to your agreements- and don’t let people who refuse to pay walk all over you. Your business agreements are no different than a mortgage or car payment. Take these situations seriously and have a process to deal with it.