I’ve been paying back my student loan via salary for 7 years but somehow owe more now than I did 7 years ago?

I’ve done 2 degrees. I checked my balance about 3 years ago and I owed around £27,000. I then checked again today assuming that because I’ve been paying around £250 per month my balance would have gone down but I now owe £33,000! Is this normal? Am I basically just paying some of the interest off or has something gone wrong somewhere?

Thanks