There are many reasons that can stop the casino from approving your withdrawal if you are playing at a UK-licensed casino.
Provided you haven't broken any rules, such as opening duplicate accounts, made large bets with your bonus or used someone else's payment details, it should all be fine.
If you have played by the rules and supplied your documents for KYC verification, you just need to get in touch with customer support to see what's holding things up.
There are numerous terms and conditions that apply when you are playing at a casino. There are many rules concerning bonuses and promotions. Casinos want to make sure that they are not giving room to bonus abuse.
If you have broken the rules by e.g. making large bets with bonus money, the operator is within its rights to withhold your funds.