Thick dirt and oily traces will cover your solar panel if you left them unmaintained for a number of years. This will make your solar panel performance reduce to half or lesser.
As long as the sunlight won't be able to reach the cell, no power will be generated.
As per the images below, the curse of these solar panel was that they were being installed side-by-side with the air-conditioner blower units. Those solar panels closer to these air conditioner blower units have been affected by the mist produced by those blowers. It has formed a thick layer of yellowish substances that cannot be washed away without affecting the cell. The cell will get damaged if you employed a hard object to remove this thick yellow substance. It could be a mixture of chemical residuals.