When the solar panels are installed it is unlikely you were told there is maintenance involved as this is not a good selling point. But here you are 12 months down the line only producing 76% of what you were when the panels were first installed.
So why are you experiencing a loss? The panels convert the suns energy into electricity so the more sun that reaches the panels the more it has to work with. If there is a thick layer of dust, dirt, bird droppings and leafs blocking the sun out the production will be down and not efficient. The solution to this problem is a good cleaning.