these pointers may sound stupid, but we are all human and sometimes common sense does not prevail.
1. make sure your skimmer is in salt water, not fresh water;
2. as already suggested by a certain reefer here, try lowering or elevating the skimmer, as well as the pump itself;
3. the air intake hose is plug into the appropriate inlet at the pump fixture. do not plug into the outlet point where the water discharges. otherwise you don't get any bubbles at all.
my $0.02 worth of experience.