Diagnosis
The system is likely air-locked, especially if you just hooked it up. Air trapped in the membrane housing or filters prevents water from flowing through properly.
How to Fix It
Shut off the feed water valve and the tank valve. Open the RO faucet to depressurize the system. Then, disconnect the tubing from the inlet side of the membrane housing (the one coming from the last filter). Point the tube into a bucket and slowly turn the feed water valve back on. Let water flush through the tube and into the bucket for 30 seconds to clear any air. Reconnect the tube, turn the feed water back on, and open the tank valve. It should start filling now.
If That Didn't Work
Double-check that you've removed all the protective plastic plugs from the filter housings and the quick-connect fittings. It's an easy thing to miss during setup and will completely block flow.
Heads Up
The first tank fill can take several hours. Don't panic if it's slow, but you should at least hear water trickling into the tank within the first 30 minutes after clearing the air lock.