Clogged line
There could be a clog in the lines feeding some of the sprinklers, or clogged up sprinkler heads.
- Remove all the sprinkler heads (the procedure will vary based on the type of heads).
- Inspect and clean the heads.
- Turn the system on.
If you don't notice any dirt or gunk coming out, and the pressure does not increase in the low flowing outlets. Cap the working outlets, and turn the system back on. If you still don't notice a difference, you might have to dig up the lines to inspect them.
Damaged line
The line feeding some of the sprinklers could have been crushed, damaged, or broken.
- Cap the working sprinklers.
- Turn the system on.
If the pressure in the low flow heads does not increase, you'll likely have to dig up the lines and inspect them.
Not enough pressure
It could just be that the system does not have enough pressure for all the heads to work simultaneously.
- Cap one of the working sprinklers.
- Turn on the system.
- Repeat.
If each time you cap an outlet you notice the pressure increase in the low flow heads, it could be that there is just not enough pressure to operate all the heads at once. In this case you might have to have the system split into zones, and only run one zone at a time.
You most likely got dirt or other debris in the line going to the faulty sprinkler head during the repair job. I would suggest removing the faulty head and then turn on that sprinker circuit to flush the line out. If there are other heads down stream from this head you should also remove those one by one and flush them as well. Finally take a good look at the "faulty" head and make sure that dirt did not get inside and cause it to malfunction. Most pop-up heads can be disassembled to permit inspection and removal of dirt and grit.
Best Answer
Here's a sketch of a typical plastic pop-up sprinkler (www.irrigationtutorials.com):
If the parts fit very loosely, especially the nozzle onto the riser, one could imagine that it might rotate a little bit and work itself looser with each operating cycle.
Rotation can occur in at least three places:
The place where the riser section fits into the body is typically splined. The screw-on cap holds the riser into the body, but the riser can't rotate because it is engaged with those splines inside the body.
First tighten the two threaded connections appropriately (the nozzle onto the riser and the body onto the supply piping). Finally, unscrew the capg, rotate the riser until it's as close as possible to the direction you prefer, and tighten the cap. Rotate the body-to-pipe connection a little bit if needed to fine-tune the direction.