5D mark ii problem

I have a 5D mark ii. I used it for an event last Friday late afternoon around 6 pm in Los Angeles. I decided to use the Program mode because the light was changing rapidly and I kept my 580 EX on TTL setting. Well, I was doing group photos but the progam mode was coming in at f-4 and that wasn't going to cut it for depth of field. So I upped the ISO trying to get the f stop to rise but it didn't work. I pumped it up to 3000 ISO but still no movement on the f stop. Am I doing something wrong?? Plus my 580 was blowing out the subjects even at -3 on TTL. ???