There are a lot of potential reasons for a late payment, and some are more difficult to deal with than others. When you notice a late payment, the first thing you can do is send a friendly reminder to your client and mention your late payment fees. In a lot of cases, this will be enough to prompt them into action.
If the situation progresses, you may need to employ a firmer tone, set a final deadline and explain that you will take legal action if they fail to meet it. If you are forced to take legal action, you can contact your freelancer’s union or your lawyer for advice and for help writing a letter to demand payment for your client