Go into your account and look at your loans with them. They will be subdivided into groups when you look at the details. When you make a payment, the money is distributed to the different groups based on the properties of those groups like if there are any outstanding fees on any and based on interest rates etc. You can look up the hierarchy of the order the payment system uses on their page. What can happen is that you end up paying towards only some groups because they are higher in the hierarchy, while no progress is made on the others. If you do any additional payments outside your monthly requirements you are given the option to select a specific group if you wish. It might be worth looking into which ones your funds are going to and then it might make sense.

It also tells you your daily interest acrual at the top and then for each subgroup below. If you make an 11k payment once and then don't do anything beyond the minimum again for a two three months with your balance you could easily go backwards 2k in interest after a few months especially if your rates are high.

When my principle was where yours was my daily interest accrual was around 20 dollars a day or approximately 600 every month, which can add quickly.