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?

TV trips GFI puzzle