There can be many reasons why a casino denies or cancels your withdrawal; the most common is probably the fact that your KYC or SoW documents haven't been approved. However, it's important to note that provided you haven't broken any terms and conditions or created a duplicate account, you will get your winnings in the end - the casino just needs some further information from you. If one of your withdrawals ever does get cancelled or denied, the casino should inform you as to why - and the steps you can take to rectify the situation. If, however, they don't, simply contact the customer support team and ask them why; they should be able to help you out.