The main culprit behind a water flow issue in an RO system is a problem with the water storage tank pressure.
If the pressure is too high, you can relieve some of the pressure by draining the water then gently pressing the Schroeder valve on the tank. If the pressure is too low, use a bicycle pump to increase it.
Clogged filters/RO membranes restrict water flow into the RO tank because they reduce the rate at which the water can be filtered.
The easiest way to resolve a water flow issue caused by clogged filters/membranes in your reverse osmosis system is to replace the old filters as and when needed.
Even if your reverse osmosis system and tank are working properly, low pressure from your incoming water supply line might be the reason why the pressure tank can’t fill.
Consult your user manual to see what your water pressure should be for your reverse osmosis system, then use a pressure gauge to measure the pressure in the water supply line leading to the system.