Check the venturi part on the pump where the hose connects. Make sure there are no calcification on it and clear of debris on the air side. Also as Bucaneer stated between 7"-10" of water is best. You might have to make a stand for it if water is too deep. Any deeper it starts to over flow in mine. I have a NAC9
Post a picture of your skimmer it might be missing something if it is second hand
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