Many reminder messages fail because they ask for nothing specific. If the borrower can read the message and do nothing, many of them will.
Ask for one clear next step
A better reminder asks for either payment or a confirmed repayment date. That gives the borrower a concrete way to respond.
Stay short
The longer the message, the easier it is to miss the core request. Keep the balance, date, and action in the first lines.
Match tone to the situation
Friendly before the due date, direct on the due date, firmer after repeated delays. Tone should track behavior, not your mood that day.
FAQ
Should I mention previous unanswered reminders?
Yes, once it becomes relevant. It signals that the pattern is visible without forcing an argument.