Zone 2 of my irrigation system has 6 pop-up sprinkler heads but the last two do not pop up, nor is there any water coming from them at all. They are completely dry, even in the flex lines they are attached to. My assumption is that there is a break in the main PVC line after the last working head. So, I have been digging up this particular line to find the break, but with no luck. It's about 30 feet long and I'm about halfway with digging this line up. I was certain the roots from a nearby Queen Palm would have been the culprit but after clearing that area with no breaks I'm wondering if something else could be the problem, especially since I am seeing no water coming out of the ground after letting the zone run for a while. So, could this be a problem with the valve or is it definitely a break that I've yet to find? Should I keep on digging until I find it? This irrigation system was not being used for at least 3 years and only recently has been turned on for the first time, so I was expecting some work to be done.
To answer my own question after solving the issue...
If the problem is only at the last two heads and the first ones are fine, then it is definitely a problem occurring after the first heads and unlikely to be valve-related (though not impossible). A broken line is most likely in this case, since your last two heads are dry in the risers and flex lines (it's not the heads merely being clogged). Broken pipes can usually be revealed by looking for muddy soil after running the zone for several minutes, especially if it's a break so bad it is completely stopping your last two heads from receiving any water pressure at all. If the soil has become impacted inside the broken pipe it may be difficult for the water to escape and reveal the break.
One common method to this problem is to increase the water pressure to the break and force the water out. It will not be so much pressure that damage can be done to the PVC joints and heads (as it will only be for a few minutes nor will the PSI be any higher than the output of your main water line or jet pump). This increase in pressure can be done by capping off all the sprinkler heads in the problematic zone. This will put more force on the possible break and on the soil that may be clogged in the pipe. After several minutes the leak may reveal itself. I'm sure there are several ways to cap off sprinkler heads, but the most common and simplest method is to buy PVC threaded caps (3/4" or 1/2" depending on your risers) that screw right on to the risers.
I agree with PJS1987. Another option is they sell a plug and play camera that would save a lot of digging and yard damage. Just plug into your laptop and stick in the pipe. Worked great for me to find a broken drain pipe.