At first, I was going to say you might have planted a loose leaf variety of cabbage, but then you threw in the comment about your brussel sprouts..
First, cabbage should be planted as early as possible. It's a cool weather vegetable. Fertilize very lightly. Over fetilization often couase the dilema you describe.
If, afte the leaves begin to form and don't show any sign of balling into a head, pull the larger outer leaves into a loose cover over the center of the. I use a large 1/2 inch wide rubber band to hold them in this position. It provides shade for the interior and keeps it cooler.
Water with religious regularity... but don't over water! An inch or so a week, but keep the soil moist but not wet. I use a soaker hose arrangement rather than getting the upper plant wet thorugh sprinkling.
True, verifiable response to hot weather by cabbages is indicated by "bolting"... sending up a rapid growing flower stalk. The plant must be pulled up and discarded if this happens. If happens early enough in the season, plant more cabbages for a later fall harvest since they are quite frost tolerant.
No one in my family likes brussel sprouts, so I have no advice on that plant...