There’s a scene in the movie Julie & Julia where Julie realizes she’s been cooking mushrooms the wrong way her entire life. Like most people, she’d been piling her pan high then wondering why they never crisped and browned like they should.
Adding any amount of food to a pan — whether mushrooms or chicken — lowers the temperature in the pan. The more food added, the lower the temperature falls. A crowded pan will cool down a lot faster and as your pan cools down, your food will stick, won’t brown and it will release all its moisture into the pan.
Try cooking food in batches, a little at a time, so it cooks properly. For smaller foods, add only enough to cover the bottom of the pan in one even layer. Larger foods, like chicken pieces or steak, should have an inch or two between them and should never touch.