Go to the Purchase History in your iTunes Store account and review your purchases to make sure that you should indeed have a greater balance left. iTunes doesn't always refresh the balance it shows, so it's possible you spent the money and it just wasn't being reflected in the display. This display error can usually be corrected by logging out of the iTunes Store and then back in.
If you confirm that you should have a larger balance left but the iTunes Store isn't showing it correctly, go here:
and follow the instructions to report the issue to the iTunes Store.