Diagnosis
The auto-dispensing system isn't delivering enough (or any) water to the mop pad. The usual suspects are a clog in the system, an air bubble in the line, or the pad itself is too dirty or worn out.
How to Fix It
Start with the pad. Remove it and swap it for a brand new, genuine iRobot cleaning pad. Old pads get compacted and lose absorbency. Next, in the app, go to your robot's settings, then "Cleaning Preferences," and crank the "Water Level" setting to "High" for a test. Also, ensure the clean water tank on the Clean Base is filled above the minimum line.
If That Didn't Work
You likely have a clog or air lock. Remove the water tank from the Clean Base. Detach the mop pad mount from the robot. Hold the robot over a sink and use the app to run a manual "Empty & Refill" cycle for the mop system. You should see water drip from the dispenser nozzles on the pad mount. If not, there's a blockage.
Heads Up
Always use the iRobot-brand cleaning solution or plain water. Other floor cleaners can gum up the internal tubing and pumps, causing permanent clogs that require service.