Credit card querry

I do not understand how this bank works, I have a credit card with them and i pay 1200 a month, and each time there is 600 or less that is available for me to use, i asked them and they gave me an explanation that doesnt make sense. last week they debited as usual, but called me a week later to say i have arrears of 317. i paid 320 and never used the card, today they are calling again saying i am in arrears of over 1000, this is a **** and it is not fair