QUICK ANSWER
First decide whether the Clean Base never starts or starts but cannot clear the bin.
iRobot allows j-series owners to trigger evacuation manually while docked. That simple test separates a trigger/communication issue from a blockage or weak-airflow problem.

INTERACTIVE DIAGNOSIS
Find where emptying fails
STEP 1 OF 3
When you manually trigger Empty Bin, does the Clean Base motor start?
On j series, you can also hold the robot button for roughly 2–5 seconds while it is docked.
Disconnect the dock before clearing blockages
Do not probe a powered Clean Base or open its suction electronics. Work only on accessible bag, tube, bin and sensor areas.