I had a few failed payment plans in the past.
Here is what I have done
I always contact buyers 7 days prior to the due date unless they have been communicating throughou the lenght of the plan.
I contact them the day after the due date to tell them that their payment is overdue and that plan was extended for an additional 2 weeks.
If not paid at the end of the additional two weeks and if the buyer faild to communicate I will send a final email explaining that due to failure to complete the terms of the plan and due to the lack of communication, the transaction is now void. (At that time the animal is re-listed and any payments made minus the non refundable deposit are returned)
The longer the plan the more you run into this type of issue, never had issue with 30 or 60 days plans but I have run into issues with 90 days plans.
Communication is key in payment plans, I am lenient but if I have to hunt you down and you don't communicate .....I won't be as lenient.
Majority of payment plans are successful but every now and than you will run into this, if your buyer has issue (yes it does happen) and if he communicates you can try to work with him, however if he ignores you than go by your terms.