I replied to this yesterday but comments were disabled.
18" drive head is about as low as you can go with most rams, you can go a little lower with different valves.
Two ft delivery head is too low, normally you need a minimum of 3 times the drive head and more likely 4 times, so think in terms of 6 ft.
Your efficiency when it is working is hopeless, with 12 gpm in and 1/5 gpm out something is wrong anyway.
I haven't seen a drawing of that design so I can't comment much, but if it uses standard plumbing non return valves it may not work at such low head.
Check what the others have said about air ( at that efficiency do you even have an air vessel)
It wouldn't dissolve its air in that time so it would have to be a leak and if that was the problem you would have to drain it each time to restart it.
You also mentioned a loop in the delivery pipe yesterday, if the highest part of that loop is the 2 ft, then your delivery head is even less.
At 1/8" drive head you may have to spring bias the beat valve towards open.
Flux