Diagnosis
Almost always an air lock in the filter housing or a kinked supply line. The new filter cartridges are packed with media and need to be fully saturated and purged of air.
How to Fix It
First, check all tubing for sharp bends or kinks, especially behind the unit. Then, with the main water supply valve on, open the dedicated filter faucet and let it run continuously for 5-10 minutes to fully flush the system and clear any air pockets. The flow should steadily increase.
If That Didn't Work
The plastic quick-connect fittings on the supply lines might not be fully seated. Turn off the water, disconnect and then firmly re-seat each blue and red tubing connection until you hear a definitive click.
Heads Up
The initial water will look grey or black from carbon fines. That's normal, keep flushing until it runs perfectly clear. Can take more than 10 minutes.