is this a new skimmer , you may have micro bubbles till it breaks in , most new skimmers go threw a break in period . once the skimmer starts to draw skim-mate those bubbles go away .
every new skimmer I got took about a week to break in , some took a little longer as each skimmer is different. the air intake may need to be finely tuned if bubbles persist after the break in.
some may experience this because the water level is low , If you have the level to low this will cause the water coming from the overflow Chanel to splash into the tank causing a splash which in return cause bubbles , I keep my water level about a 1/4 inch from the overflow Chanel and never get bubbles , higher water levels keep the sound of running water down also ,