I believe that happened because you have not completed wager requirements. Whenever I won some contests by Whale and received my bonus, I also got a message fro, Whale about wager requirements. In my case it was usually was 1.5. Only after completing them you I was able to withdraw my winnings (if it was so, because many times I lost my prize chasing wager requirements).
This was the first thing that came to my mind. As far as I have had experience with Whale and withdrawal from their platform, I got the impression that they have the fastest withdrawal of all the casinos I have used. An incomplete withdrawal can only mean that some of the requirements have not been met, and it is probably possible that the wager requirements are involved.