Welcome to the Vine HIL. Brassicas / cabbage like firm ground and the conventional wisdom says they won't heart up in loose ground.
BUT - last year I grew them in new raised beds. To begin with they looked like yours - loose leaved - then they did heart up. If you only planted them in April there is plenty of time for them to heart up. Patience solves most problems. Good luck.

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From each according to his ability, to each according to his needs.