We all know from our high school physics classes that it takes a lot of effort to get an object in motion. Once it gets going…not so much.
Doesn’t matter much whether we are riding a bicycle, driving a car or…trying to get a customer out of the habit of paying several days past the due date and still taking a discount.
If you customer does not know your policy or you do not enforce it (reasonably) then you don’t have one. It’s not entirely their fault if you have been ‘shy’, so you can call or send a letter: “We realize that perhaps we were not clear in the past about the terms you agreed to so we thought it best to”, and then when your policy will be enforced.
Starting the motion involves friction, but it gets easier once you begin contact and dialogue to ‘move’ the DSO lower.
As the noted Australian philosopher O. Newton-John once said, “Let’s get Physical.”