collection company has added to principal amount on student loan

by KR
(Portland, OR, USA)

I am ashamed to say that I have an outstanding student loan from the 80's. My federal tax refunds have gone toward paying this debt over the years, but the principal on this loan has gone from $700 in 2000 to $3450 as of last year. The debt has been transferred to several collectors over the years, but it seems they have added penalties and interest to the principal. I broached the subject with the last collector I talked to and he ignored it. I asked him for the records of how the principal had been added and nothing came of it. Can they do that?