Usually the LED and the dimmer disagree, or the load sits below the dimmer's minimum.
An incompatible LED, an overloaded dimmer, or the wrong dimming type for the driver.
A tiny leakage current through the switch or the wiring can light an LED that is switched off.
A light dimmer is listed for lamps, not fan motors, so the fan needs a speed control and the light kit needs its own dimmer.
LED drivers draw a short inrush spike that can weld relay contacts so the lights stay on.
A constant-voltage driver and a constant-current driver are not interchangeable, and the dimming method has to match the control.
Check time delay, sensitivity, coverage, and mode before you replace the sensor.