In a small academy or a school office, the person who has to ask for the money is the same person who coaches the child, or greets the family at the gate on Sunday morning. Asking turns a warm relationship into a transaction for a minute or two, and nobody enjoys being the one to do it.
So the ask moves to next week. Next week it moves again, because now it is bigger and more awkward than it was. Meanwhile the parent has heard nothing at all, and silence from an organization reads as "everything is fine". They are not avoiding you. They genuinely do not have the invoice in front of them.
By the time anyone says something out loud, two things have gone wrong at once: the amount has grown past what a family absorbs in one payment, and the conversation now carries an accusation it did not need to carry.