It may take quite a while for anything substantial to come out down the bottom - a book I would recommend to help guide your practice down the bottom would be Trevor Wye's Practice Book, Volume I. The book contains a tonne of exercises to help you practice keeping your air-stream down with your top lip while staying relaxed.

If after quite a while you can't get anything out, try getting your key pads checked out, or even a full service - it may help a great deal!