In business you will always find that you are dealing with slow paying customers. Unfortunately there is no escaping this fact.
So how do you deal with slow paying customers effectively?
At the end of the day, the answer is the same as every other situation. You need to treat each customer's situation as an individual situation. Of course you may discover that specific types of problems occur. The problem which one applies to the customer account in front of you at this moment, may not be at first obvious.
Therefore you need to discover the cause of your customer's slow payment. Only once this problem is discovered, can you create a strategy to deal effectively with that customer's situation.
If you have not tried this approach of contacting each slow paying customer previously with the view of discovering their problems, try now. I believe you will start to see a different picture of your trading relation relationships with your customers.