I recently installed 4 GFI receptacles on a 20 amp circuit with a GFI circuit breaker. The circuit breaker kept popping and I traced the problem from the circuit breaker to the end, and realized that there were no shorts and the GFI circuit breaker didn't like it that I wired them all on with pigtails on the line side. I re-wired them all, going in on the line side, out on the load side, in on the next line side, out on the load, and so on, where each GFI was on the load side of the last one. It worked fine after that. I thought it was strange that the circuit breaker didn't like it when I wired it with pigtails.
