As Jalexa has said the it is that you are probably only paying the minimum amount and if this is the case there are two problems:
1) This shows on the enhanced reporting on your credit record (They can now see how much and a marker is used to indicate if it is a Minimum payment, and also another marker showing if you are on a promotional deal (0% etc).
2) You will take an extraordinary time to pay this debt off.
A guess at the breakdown of the payment into repayment and interest would be:
30 - 35 interest (assuming a 15-20% APR), and 5 - 10 repayment.
So on a reducing basis it would take a rough estimate of 75 months (15%) to 110 months (20%) to repay at this level!
Repaying your debts (clearing the card) is a positive thing for your credit record, and will show that you are able to handle debt.
HTH
Sparky.