There are two types of transactions that are involved with your installment plan.
First is your monthly installment payment and this amount is always the same. It is charged immediately upon plan creation, and will then be charged automatically each month.
Second is the authorization for up to the full purchase price. The authorization is a hold to guarantee your future installment payments and will not be due on your credit card bill.
Both of these transactions must remain on track in order for you to continue paying in installments.
Want to know more? Check out this article on payments versus authorizations.