It is much more likely that they backfilled the head with sand and you are seeing a area with a high perk rate. If there is any doubt place cups in a line every 2 feet and let the heads run for 15min, the cups will indicate the precip coverage.
If you have dry areas with matched precipitation then you need to ammend the soil, not the head. Try core-airaiting remove the plugs and sweep soil ammendment into the core holes.
The harder I work
the luckier I get.