Unless businesses make it easy for customers to provide a suggestion or complain, most will not bother; they will simply take their business elsewhere. Thus, businesses need to both encourage customers or clients to contact them and make it easy to do so.
Firstly, eradicate the common belief that businesses do not want to hear about problems. Make it clear that you genuinely want to help by sending the strong message that the company can only solve the problems that it hears about.
Furthermore, the actual act of complaining must be effortless, or customers will not bother to provide feedback or, much worse, post a complaint on social media, which directly affects the reputation of the business.