Unfortunately, many, if not most states have adopted the policy of charging Probationers for being on Probation. Each state charges different amounts for each month you are on Probation. They claim if you use the system, you must pay for it.
While I personally do not agree with this policy, it nonetheless exists and you are bound by it.
If you still wish to fight it, speak to a local lawyer who wants to start a class action against the authorities. I believe if the public is required to pay for schools for everyone, regardless of whether they have any children in them or not, then the same should hold true for the public paying for Probation and prison. Its a public facility and the public should pay for it, not just those who go through it. The problem is the public would rather have probationers pay for it and share the expenses of all schools equally rather than forcing those with more children to pay a larger share of school expenses. Someday it will backfire on the public and those who have no children will make the same argument in reverse that they presently make about probationers.
Unfortunately the answer to the question of being behind on your fees is likely to be that you knew what your fine and fees were - they were explained to you the day you got your probation. You should have been aware that you were not being charged the proper amount and you should have questioned this but you didn't.
You could hire a lawyer to talk with the judge about the problem. The judge could terminate your probation or could at least not assess the probation fee during the extra months they added for you to finish paying the arrearages. However, hiring a lawyer is likely to cost as much if not more than the additional costs of probation.