Diagnosis
The water isn't entering the drum. The most common cause is a clogged or kinked inlet hose from the steam generator.
How to Fix It
Pull the dryer out. Check the small, clear plastic water inlet hose at the back. It runs from the steam generator to a solenoid valve. Disconnect it at both ends. Check for kinks and blow through it to clear any sediment clog. Also, ensure the water supply valve behind the dryer is fully open.
If That Didn't Work
The steam generator itself can fail or the water inlet valve solenoid can be stuck. You'll hear a distinct click when a steam cycle starts if the solenoid is working. No click means a bad solenoid or control board signal.
Heads Up
Only use the dedicated steam hose. Don't substitute a random tube. The steam generator uses a precise amount of water. Too much can damage it.