Problem with home waterworks, flow meter is losing pressure

Hi, I am a bit desperate. I had already bought a cistern with a suitable domestic water system for garden irrigation from Graf some time ago. The installer who does the house installation was supposed to install it. Now, everything has dragged on and the installer only sent subcontractors, so the communication and so on was very bad and in general, we have pretty much fallen out over the whole situation. I have therefore done the outdoor distribution myself with the garden pipelines from Gardena and the connectors, etc. The cistern pump sucks water and also refills fresh water into the refill console. So far, I would say everything works. The flow meter (Control Matic RMC E) controls the start or stop of the pump when taps are opened. Now I have the problem that the pressure from the flow meter is not maintained when the taps are closed. The pressure steadily falls and eventually reaches zero. The flow meter then goes into error and does not start when an outdoor faucet is opened. That is of course crap. I cannot reset the pump every day. So I probably have one or more leaks in the outdoor water lines? I have checked the part I can still access and see nothing. Of course, there are also a few elbows under the driveway, unfortunately I can no longer reach them. Maybe the plug connection there has become leaky afterwards?!? I have already checked the check valve in the cistern, nothing seems to run back. The union coupling between the pump and suction hose is brand new and also seems to be tight. I also see no air in the suction hose. I don’t see any error there for the time being. So rather on the water consumer side. What I do not understand at all now is why the pump does not start to increase the pressure again? I mean as long as I have water in the cistern it could basically not matter to me if the pump starts from time to time to build up the pressure again. Or is there perhaps another flow switch that can be better adjusted? Maybe someone can help me. Thanks

We have a similar system:

For us, it runs smoothly. At the beginning, we also had the problem that there was occasionally a drop in pressure and errors with the pressure switch: The solution for us was ultimately on the suction side. No problems for 2 years now. (If the pressure side were leaking, the pump would simply start regularly as soon as the pressure switch threshold is undershot, even if all consumers are closed – but it would never completely drop out)

I would also initially check the following with you:

[*]How long is the suction side (cistern to domestic water system)? It should be max 10m long
[*]It must really be tight – from the floating intake/check valve up to the pump – so check all connection points and clamps
[*]Regarding the GK bayonet coupling before the ball valve, I’m not sure if it is also suitable for suction/negative pressure or if minimal air might be sucked in there (which you don’t see as a leak). I would remove it and then test: so instead of ball valve – claw coupling – suction hose new: ball valve – hose nipple – suction hose

edit I quote my Google result on that:
A standard coupling seals by twisting the bayonet connections against each other and is not vacuum tight, i.e. not suitable for the suction hose.