RO/di machine headache

[hillegom]

Is that 120 psi before the prefilter, or like you said, just before the membrane?
Something is wrong with the membrane if you have 120psi just b4 the membrane but you are only getting 10 gal per day. Try back flushing the membrane.
What is the psi after the membrane? What is psi before membrane?
Possibly check your plumbing.
When I change my prefilter for the second time, I will change the carbon as well. The carbon wont clog as fast if your prefilter micron rating is smaller than the micron rating of the carbon.
The ro water to waste should be around 1:4
If your filters are all new and you have 120 psi, then you should get at least 100 gal/day from your system. After using for a while, the filters clog and u will get less