All RO membranes are measured to an industry standard in US gallons which is tested at a water temperature of 25C and 60psi (4.5Bar) pressure. In reality the temperature and pressure in normal tap water is never as high as this and so you will see reduced water production.

A reduction in water temperature from 25C to 10C will reduce the flow by about 33% and a reduction on pressure from 60 psi (4.5Bar) to 40psi (2.75Bar) will also reduce the flow by 33%. If both cases exist then the flow will be much lower.

It is also worth checking that the RO is intalled correctly, you are running the correct flow restrictor for the RO membrane you have and that any flush valves are closed. Booster pumps are available to increase the water pressure to your RO unit, these will help where mains pressure is low and can inprove the overall performace of the RO membrane.