My first diagnostic would be to remove the sprinkler head and turn on that zone. If the water shoots about 10' foot in the air you have good pressure. That means you have a bad or clogged sprinkler head. If you do not have good pressure, it's an issue with pipe in the ground or a bad valve for the zone you're working on. If you're handy, I'd replace the valve for that zone first which is a lot less costly that fixing a bad pipe issue.
Most new homes in TV use the rotating Hunter sprinkler heads which I believe are close to useless. I've replaced all my sprinkler heads with fixed heads.
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