Almost all issues related to custom debt payoff order have to do with the deferred interest option on the debt details page. A debt account that is truly a deferred interest account needs to be paid first regardless of plan, so these debts are automatically put to the top of the list.
If you are not using the deferred interest option, check the payoff order of the debt on the details page. Make sure another account does not have same payoff order number assigned to it. If you have an Undebt.it+ membership, check out the Payoff Plan > Custom Payoff Order page that let's you drag & drop.
NOTE: The deferred interest term can cause some confusion. This is not the same as a loan that is deferment, like a student loan. A deferred interest account is one where you will pay the accrued interest if the balance is not paid by the end of term. For example, if you buy an appliance at Best Buy with a "120 days same as cash" promo, it's a deferred interest account. The standard interest rate on these accounts is almost always very high and if you don't pay it off before the term ends, you will get hit with the deferred interest.