After failing to make an on-time payment, the a person will owe their regular current child support payments plus the past-due child support owed. The same is true if a person fails to make on-time alimony or cash-medical support payments.
For example: You have been ordered to pay $200 per month in current child support, and you fail to make your first payment on time. The next month, you'll owe $200 in current support plus the $200+ owed in past-due child support.
Conversely, past-due child support is the amount a person is ordered to pay if they didn't make a current child support payment on time.
Depending on your situation, you may be able to change (modify) the amount of current child support you are paying. Generally speaking, it is much simpler to change the amount of current child support owed than it is to change the amount owed in past-due support.