Unfortunately you may have fallen into a pattern with these clients in which they anticipate that you will tolerate late payment. In some cases, they may not be managing their payables properly. Regardless of the reasons for late payments, at the end of the day, you have rendered a service and not been paid...not good business for you. A payment structure that provides either ( front-loaded) staged payments, OR a meaningful financial incentive  for compliance with a payment structure that front loads part of your fee may improve cash flow for you. Alternatively, a "preferred" rate can be offered for entities that pay on time.