Troubleshooting Reset Issue with GFCI Breaker in Secondary Panel?

I hooked up two 6 AWG stranded wires and a neutral from a 60A two-pole breaker at the main panel to a secondary panel in the shed 30 feet away. I ran a ground wire to an 8 foot copper rod for a good ground at the shed. I am trying to get a 20A GFCI breaker on the secondary panel to work. When I apply power at the outside receptacle connected to the GFCI breaker, it resets. A compressor on a separate 20A two-pole circuit at the secondary panel works, along with 110V outlets in the shed. Am I missing something about GFCIs?