There are several reasons why your withdrawal request was not approved. If you failed to complete the verification process after registering your account, you will not be allowed to withdraw your money. If you have a bonus or free spins offers, you need to meet the wagering requirements before you can withdraw. It is also possible that you did not meet the terms and conditions.
If none of these check out, look at the information you gave. Maybe you gave the wrong information in your withdrawal request. For instance, you gave the wrong name or placed an amount that is higher than what is in your balance.
If your withdrawal is still not approved, get in touch with customer support.