I have nine 3/4 inch pop up heads in zone 2. I have replaced a number of heads and it seems my coverage gets less and less. Now most of the heads don't fully deploy and a few hiss and fizzle. Last night I went out and bought 5 new heads (rainbird) and replaced the back yard heads. I turned the zone on and none of the heads deployed more than an inch. The pump is five years old and every other of the five zones works ok. I can't see any leaks and don't know what I can do short of calling the company who put the system in. Any ideas or tips folks. Thanks C