I am not familiar with the design, but 18" is about the minimum drive head and at that head you may need a valve that is spring or weight biased to open.
Your outlet head at 24 " is incredibly low. normally the delivery head needs to be a minimum of 4 times the drive head. If your output of 1 gal/ minute with an input of 12 gal/ min is at 24" then you are doing very badly.
With a good ram and a delivery head of 4 times the drive head you should manage about 70%.
First thing try increasing the delivery head and if that works and you only need 24" then you could use a restrictor to artificicaly increase the delivery head.
I don't like the loop in the delivery pipe unless you have a small hole at the top of the loop to prevent syphon. With the hole you could use this to increase the delivery head.
If by increase head you mean drive head then you will need to increase delivery head in proportion.
It would help if you made it clear which head you mean and what delivery head you need.
Flux
