Mineral scale buildup inside the steam valve is a common cause. It needs descaling and lubrication.
Run a full descaling cycle using De'Longhi descaling solution. After descaling, with the machine off and cool, turn the steam knob back and forth several times to work the solution through the valve.
If that doesn't work, the internal O-rings in the valve may be degraded. The knob mechanism itself may need replacement.
Never force the knob. You can damage the internal seal. Always turn it gently to the fully open or closed position.