I settled with collections for my Sallie Mae defaulted loan...?

But my credit report still says I owe! The collection agency left me settle for 50% of the amount due (after I made a few payments.) I did this, and then ANOTHER collection agency demanded the other 50%! I faxed them over the settlement information (which included the letter offering me the 50% payoff and proof of my payment.) They never contacted me again. Now my credit report, after three months, STILL shows that I owe that other half! I'm sick of it, and Sallie Mae keeps giving me the run-around! What do I do?