Alright, sorry for the delay but here's the update :
So I finally figured out what was going on, and I feel like a bit of a noob!
After searching for a leak, I replaced the diaphragm because I was sure that was it. Obviously, that didn't fix a thing.
Now you have to understand that this is a pretty large grassy area full of St. Augustine grass, and there's way to many heads on the zone to begin with, but I wasn't involved in the install. So I did a walk around of every square inch of the area and found 3 heads with St.Augustine runners over the top of the pop-ups that allowed them to rise up just enough to have the water pour out from between the riser and the body, which are all basically leaks. I guess that those 3 issues, coupled with the over-abundance of heads on the zone, resulted in the low pressure situation.
So that's the story. It was hard to find because there were not really any brown spots near the malfunctioning heads. It all works fine now, so mission accomplished. Thanks for the help everyone