Hi sulli,

Do you know know the system's age? Does the tank become pressurized when the pump runs? Is there a hose fitting or faucet from the tank? Does each zone run separately? Do you know the flow rating of each head and the well's flow rate? The sum if the head rate should equal 80 percent of the well's flow rate. I don't think is your problem but worth a calc.

#1-Measure water pressure of the mainline from the tank or from the tank itself. That is the maximum any zone will be running by itself. This can be done by buying and installing an inexpensive water pressure gauge from a hardware or irrigation store. You can also test your house water pressure the same way. If you want to test each zone, replace an irrigation head with the pressure gauge and run the zone. The pressure will be a little higher because one head in the zone is missing. I can give greater degree of detail detail commensurate with what you feel comfortable.

#2-The pressure can only be as high as the pressure of the mainline source. Pressure can be increase by installing an inline booster pump($$) or change your pump for one with more pressure lift($$).

#3-Yes. Anything with moving parts wear out. Try removing the head and soaking the entire head in vinegar for an hour or lubricate moving parts with waterproof lubricant. If you don't want to be maintaining continuously then replace with new one of high quality($$).

#4-probably not but give it a shot. Remove the last head in a zone and connect a pipe with a right angle to shoot water away from the area and let it rip. This gives good scouring velocity inside the pipe.