I installed a GFCI outlet in my kitchen to replace the outlet closest to the sink. There are two other outlets "downstream" from the GFCI, such that when I hit the test button on the outlet, the other two lose power as expected. I have tested them with a tester which has 3 lights to indicate open ground, open neutral, open hot, etc, and it lights up as Correct. I have also verified they are connected per the diagram provided with the GFCI outlet, such that the incoming Line power comes from my breaker panel (20A breaker), then the GFCI's Load goes to the Line of outlet B, and subsequently the Load of outlet B goes to the Line of outlet C. The wiring at the GFCI matches the wiring diagram provided with the GFCI for two cables entering the box with the exception that there is no grounding terminal to the box. Up to this point, all appears correct.
The problem is that when I plug my microwave into either of the two outlets B or C, the GFCI trips. When the toaster is plugged into either outlet B or C and is turned on, the GFCI trips. When I plug the microwave directly into the GFCI outlet, it works fine.
What did I do wrong?
Could the problem be with the gauge of the wire?