Apple ID balance not updating

If you didn't owe iTunes money, and/or have pre-orders/subscriptions due, contact iTunes Support and see if they can see what happened : http://www.apple.com/support/itunes/ww (click on your country's flag, the country where you live, and which is therefore the country on the account ; click on Contact Support towards the top right ; click on Get Help under 'Talk To Us’)

Assuming that it was successful and that you didn't owe iTunes money, have you tried logging out and back into the account ?

Have the pre-orders been released today and therefore now bought ?

If no pre-orders that you had were released today that won't apply in your case - you know that I'm just a fellow user so obviously have no idea what has or hasn't happened on your account. Contact iTunes Support via the link I gave.