You Aren't Tenderizing Your Meat
There are a few different ways to tenderize your meats. Lean meats, particularly lean poultry, need a little help because they don't have the same intramuscular fat that other fattier cuts do. By either marinating that meat in acid like lime juice (which breaks down the tissue over time) or pounding it out so it covers more surface area and breaks molecular bonds, you will make a much more enjoyable bite.
You Check on Your Food Too Often
Every time you open the oven, heat and pressure get released. Your oven then has to self-regulate once again to bring itself back up to temperature. Avoid opening your oven when you're cooking, just use the little window in the front.