They need to break in for a while, mostly to get rid of residue left behind by the manufacturing process. This can take hours, days, weeks, it really depends. You can speed the process up by giving the skimmer and all its components a thorough cleaning (no soap, though) before you set it up and start running it. Other than that, it depends on what skimmer you have and what adjustments may need to be made to get it working properly.