Tankless on gravity hydro electric system

Trying to figure out why newly installed tagaki tankless won’t fire think it’s to do with internal pressure sensor.

I have a 2-1/2” pvc pipe to the hydro electric system which branches 1” as it passes cabin for domestic water. In crawl space that water main has three Feeds.
the first 3/4” pex up to kitchen sink, then 1” pex up for Tagaki where I’ve got 40lbs showing on a gauge (and a 3/4” tee to shower cold). The end of the water Main pipe under house comes up 1” pex in bathroom wet wall Ts into 3/4” both directions to laundry toilet and sink.
There’s 60lbs showing on the gauge in the hydro shed while the hydro is running. So about 130’ net head. The house is higher thus the less pressure.
When I turn on COLD from the first ( kitchen) or last ( bathroom) branches house gauge doesn’t move from 40 lbs.
But when I went to commission tankless unit by opening cold water valve into it and then opened hot water valve out of it pressure dropped to zero took a few minutes to recover.
Figured needed to get air out so opened each of the outlets water air surged but water quickly tapered off, but purged the air that’s not the problem. checked for any open lines I “ forgot” not possible it’s all accounted for mostly visible besides it rises 40 psi after a while.
Tagaki says next open a faucet to verify flow through unit then close it again. Pressure drops I think ok I get that pressure drops from open faucet ( even though doesn’t drop when other water main branches are opened?) but don’t get why water flows tapers to drizzle after a minute oughtnt it at least flow as well as cold water even if tankless water not firing isn’t cold water simply flowing through unit?
So attempt to ignore and fire unit. Propane lines purged for sure. Tagaki only reads standby.
So I’m thinking it’s a pressure sensor inside unit then think but all water system are pressurized by city or pressure tank/ pump and all would drop when hot faucet or cold faucet opened I’m just hyper aware of it because I thought I’d be clever and install a gauge in house to monitor hydro system.
What am I missing.
I used this same hydro electric gravity system on previous cabin with a tank type water heater. 40 lbs ought to be adequate no?