advice re: low sprinkler pressure--brown patches on lawn

One particular zone of sprinklers have always had problems with low pressure--especially after we've had a landscaper come out to cap some sprinlers while they install a concrete walk out to the driveway. At the time, the house was still under warranty and I had the developer's landscape contractor come out and look at the sprinklers. Their comment was that zone did have a lot of sprinklers in it--just increase the duration the sprinklers are on in that zone.
Well, one year later, I discovered that the lawn has started to develope brown spots and the sprinklers are not popping up enough in that zone--an indication of low pressure. I did clean out one of the sprinkler head filters and did not see any dirt that might be causing low pressure.
What else can be causing the sprinkler heads to not pop up enough? Any comments or thoughts will be greatly appreciated. Thanks!
Lil
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It could be a flaky or mis-adjusted valve, it could be a partial blockage, or it could be a leak somewhere in the line.
steve
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