depending on your Ca levels you may need to clean your
skimmer pump. Ca deposits like to build up on the pump motor making them less efficient. how is air delivered to the skimmer? you should also be cleaning the air intake whenever you clean the collection cup.
as nutrient levels in a tank changes the skimmer will also need to be adjusted. this is even more so if you have a sand bed. the nutrient levels in a sand bed are in a constant state of flux. some times it is releasing more nutrients than it is absorbing. this can cause the skimmer to behave differently between weeks.
you really can not have a skimmer that is completely adjust and forget.
G~