Well it could possibly be a few different reasons. Cooling time If you cut your sourdough while it's still hot, steam trapped in the crumb can make the interior wet and gummy. Oven temperature If your loaf is golden brown on the outside but wet or gummy on the inside, it might be baking too quickly. Try reducing the temperature and baking for longer. Over-fermentation Over-fermentation can make dough wet and sticky, and prevent it from rising properly in the oven. Let the dough rise until it's about 20-30% larger and you start to see bubbles forming Maybe your starter wasn’t strong enough? How long do you leave your bread in the oven? & do you wait for it to completely cool down?