I am guessing here, but it is about all I have got so far. I would guess that since they are all installed on the ground level, they are draining when the system is turned off. When the system is turned back on, the water / air rushes through and causes the adjustable emitters to self adjust themselves / or give a spitting sound or effect. This can give the the false since on the emitters not having adequate pressure / or flow. I would suggest pressure regulation on all of your low volume / drip lines, and allow adequate time for the system to stabilize before checking it again (turn on and let run for about 10-15 mins). Most low volume systems are just that, real low volume, and they need more time to deliver unlike their standard irrigation counterparts. Its a stretch and its a guess!
Josh
Irrigation /Landscape Lighting / Pump and Well Specialist