TV trips GFI puzzle

I recently added a standard outlet (4th) to an existing line of outlets after a GFI on my deck. This is for a TV and cable box. When I plug in the devices, it trips the GFI. I'm 99.9% sure the wiring is clean and correct. I didn't use pigtails. When I plug in the devices to the third outlet there are no issues. When I did this I used an extension cord that has three prongs. The TV plug is only two prongs. Could the two prongs be the issue causing some sort of ground error?