You must use a pump that has enough pressure to produce venturi action, at the amount of head pressure it has to overcome (the height of the mixing chamber.
There are many, I have found that for my skimmers a 950gph pump is about minimal. Mag drives, Laguna have worked for me, but I also build DIY skimmers as opposed to using commercial ones.
There are pumps that have a special apparatus that chops large bubbles into small ones, in order to produce a more effective foam.