There are several reasons why an account may not appear on your credit report.
There is actually no requirement for a merchant to report your account to credit bureaus. Most financial institutions and lenders do report to credit bureaus, but may not report to all three of the major credit bureaus. If you have questions about whether or not your account should appear on your credit report, you should contact your creditor and ask which bureaus they report to. Similarly if you are trying to build credit, you should inquire with the bank to make sure the bank reports to all credit bureaus before opening a new account.
Another area of confusion can be when you open a new account. Creditors can often take up to 60 days to begin reporting your information to the credit bureaus.