Check that the airline is clear. There will often get to be calcification where the air meets the water in the venturi (the bell shaped thingy on the end of the pump). You should be able to remove the venturi and soak it in vinegar overnight. You can also pull the airline off and inspect that the opening is clear. A partially blocked airline will produce the effect you describe in your first post.
On another note, a person's opinion of the ability of a skimmer is directly relative to the other skimmers that person has used. If the person who sold you the skimmer never tried anything better then it is quite possible that he indeed thinks the skimmer "is very good". As with everything in this hobby, buyer beware - research potential purchases
before purchasing.
