When you run, your arm acts as a counterbalance. The arm is also a pendulum - it swings back and forward. The time it takes for a pendulum to swing depends on its length. So the lower your hands are when you're running, the slower your arms will swing. Likewise, the higher your hands are, the faster the swing. To have a good Cadence you need to have your arms held reasonably high. If you are struggling to keep your Cadence high, check your arm position.
Below you can see two pictures, one with the arms too low, the other with the arms in a better position. They key is not to mimic a specific arm position, but rather to keep your Cadence high, and raising the arms can make a difference.