Lentils are fine. Pasta is also good. Like everything, it's all about having the right amount of water and the right cooking time. You have to allow enough water for the food to absorb water and for the steam to build up pressure in the pot. If you don't allow enough water for both, your food will dry out and stick to the bottom of the pot, because the pot has to come to pressure - it's what it's programmed to do. So it will keep heating in an attempt to make the steam that will raise the pressure to seal the lid. Eventually, your food will get dried out, stuck to the bottom, start to burn and the pot will send an overheat error and refuse to do any more.
So, when you cook foods that absorb a lot of water and expand, you have to calculate how much water they need, then add an extra ½ cup or so for the pressure cooker itself. When you're cooking foods that release liquid, you can start off with just a little bit of water to get you going, because you know that as it heats up, water will come out of the food and provide the pot with the liquid it needs to create the steam.
If you're cooking with high sugar content foods (e.g. tomatoes), make sure there is something as a barrier on the bottom of the pot to prevent burning. For savoury dishes, onions are great. Don't be tempted to stir things through like you would normal food. Layer it.
Pot in pot is a good way of cooking. Lasagne in an 8" cake tin is great.