Whole Wheat Sourdough Loaf

Uses only 100% whole grain King Arthur whole wheat flour, water, and whole wheat sourdough starter. First rise can take 24 hours. After baking and resting, needs to be cut and used or frozen almost immediately, or it gets really hard and dense. Otherwise very nice.

Question - the inside feels sticky when freshly cut even though the digital temperature probe read the right temperature for it to be done. It's not raw dough, just sticky. Am I doing something wrong? Also, how can I make it fluffier, or will that require mixing in white flour?