If it's a new install, given that they didn't use head's pulled from a revamp, then they probably flushed a bunch of crap into the line. The lack of coverage could also be from improper pattern adjustment. I would advise the following steps, to see if the situation can be fixed in a " cost effective manner ".
1) Pull the head from the fitting, then use a 3/4 or 1/2" coupler to a piece of polypipe ( 3' or so ) to flush the line.
2) Take a 5 gal bucket, half full of water, and disassemle the head. Then see how much crap comes out.
If the heads are clean, then they were probably pulled from anouther job and are worn out. The other possibility is that some dumbass decided to squirt WD-40 in the collars to " keep um loobed up ", different means to the same end.
If there is substatial dirt/crap in the head then rinse the heads, flush the line's and check the pattern adjustment. If the aforementiond doesn't work, then it's time to call in a reputable irrigation contractor to asess the problem.
Best of luck,
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