I was called into a large commercial site where the LA swore he saw the heads pop-up during the walkthrough and failed afterwards. I could only figure that air in the lines gave enough pressure to pop the heads the first time, the second time the main was full and the heads failed and sat at complete flow-by. That was a good job for us, given the dipwads had not given us any spare wire.
I had to learn this lesson the hard way. Did an install this spring where we had about 45 4" popups in a flowerbed and they were all 5' variety. We couldn't get the zone to pop up and seat. I only had an idea of what to do after my boss put a bunch of Hunter 2' h in his parking strip and checked valved half of it. So I threw about 25 check valves into the furthest heads and it worked. The customer saw it not come on the first time "why no work". Just an adjustment issue ma'am