Remember, a debtor is someone who owes your business.
The debit to bank means more money for your business and the credit to debtors means our debtors (also an asset) are decreasing - meaning less money owed to your business.
Now, when you discover that the check has bounced or is bad for some other reason, you should reverse what you did originally.
The journal entry would be:
DR Debtor CR Bank
What you are showing now is that the debtor owes you the money again (debit debtor to increase it) and your bank account is back to where it was before (it's less because you didn't actually received the funds in the account in the first place).
Note that the Accounts Receivable account is often used instead of Debtors. This basically means the same thing: the value of debts owed to your business.
Did that explanation make sense? Have your say in the comments section further below.