you for reaching out to the Community regarding your issue, although I am sorry
to hear that you are experiencing issues with your Sallie Mae account. Is the interest payment the only transaction missing? I'm wondering what might of caused it to be the only one not to import. Is this the first time this has happened? One thing we might be able to attempt to see if we're able to get it imported is to download a QFX file from Sallie Mae and import it into Quicken.
You should be able to do so by going to Sallie Mae's and signing into the website. From there find an area where transactions are downloaded/exported and export to Quicken. From there you'll want to import it into Quicken which can be done by opening the file and selecting the correct account to link it to. The article below goes into detail about how to download the file.
you get a chance to try these steps, please let us know what you find. From there we'll better understand our next