There's a good chance your system demands more water than your line can carry. Another symptom of this is that you find that some watering devices throughout the system drip but some do not. You will need to add the total flow rates of the water dispensing devices on the line. For example, 10, 1 GPH emitters would use 10 GPH
1/2" line can carry a max of 240 GPH
3/4" line can carry a max of 480 GPH
1" line can carry a max of 780 GPH