When I hit the start button after selecting pressure cooking, nothing happens. What's wrong?

Assuming that your control panel is lit up and the appliance is plugged in, probably nothing is wrong. Think of the inner pot like a pot on the stove. You put the pot on the stove filled with food and liquid, you turn the burner and on and nothing changes in the pot. Same with the Ninja Foodi. As long as your control panel either has dashes going around in a circle or the letters "PRE" on the display, everything is fine. 

When you hit the start button, that is like turning on the burner on your stove. The heating element in the base turns on and begins to transfer heat to the inner pot. Once the pot starts to heat up, the liquid will start to boil and that will produce steam. Once enough steam is built, the red or silver pin on the pressure lid will pop up meaning your pot is now under pressure. Within seconds to minutes, the dashes will turn to the pressure countdown or the "PRE" will switch to the countdown. 

The time it takes to build the pressure will vary based on the amount of liquid, amount of food, and the temperature of the food. It averages about 8 minutes, but I've seen it take 20+ minutes. 

If, after 20 minutes, nothing has happened, then it's time to troubleshoot to see why the pot won't build pressure.