In my years of experience using the Instant Pot for pressure cooking, there are two main causes of tough and chewy or dry chicken. The first is overcooking. Chicken thighs just need 6-8 minutes of cooking at high pressure to come to at least 165ºF. The second cause is using the quick release method right away when the pressure cooker cycle is complete. Always use a natural pressure release (leaving the pot alone after the pressure cooking is finished) for at least 10 minutes. This helps keep the chicken juicy and tender.