If it is happening after 15 minutes, every time, then it's likely a pressure issue inside your shoes. Your feet swell up with activity, so when you tie your shoes at the beginning of a workout your feet are one size, and 15 minutes into the workout, they have begun to increase the blood flow in your feet, and your feet get bigger. When they are bigger, the laces that felt fine before your workout will be tighter.
Try lacing your shoes differently, and if that doesn't help go see a podiatrist.
HTH