We have a LG electric tumble dryer. Sometime when pressing the start button the dryer will hum and not tumble. Two things get it working again normally:
1. Turn the drum by hand during the humming to sett it tumbling.
2. Unplug it and leave it for several hours, and then it restarts normally.

Troubleshooting steps taken so far:

Disassembled the dryer and vacuumed out all the lint around the components.
Checked the belt. It's not worn.
Checked the blower fan for debris and that it spins freely. No debris, spins freely.
Checked the belt tension pully. It spins freely.
Checked the rollers. They spin freely.

We're puzzled as to the cause of this intermittent problem.
A bad motor? A bad control board?
Any ideas welcome.

Update,
Today dryer wouldnt tumble. Removed lid to find belt turning and the drum slipping beneat the belt. Is this is a worn belt? Also noticed the belt had moved toward front of drum away from original alignment on the drum. Once the belt pushed back to its original spot the drum tumbled and all seems fine now.
Is the belt worn-out?
What causes the belt to move out of position?