Re: why myself cannot use debit card for apple itunes payment?
I've has similar issues with the "odd" declining despite there is enough funds available and in my case, i just left it, because i know it will eventually go through, issues can also happen Apple's end as well, but if its an on going issue, then speak with your bank.
If you phone up Apple asking then, they'll only refer you to the bank anyway..
To prove, its card related, try another card with Apple. If your bank says everything is fine, then speak with Apple, as don't rule them out.... they have their own problems too with debiting.