A possible cause, apart from inexperience, is that your mouthpiece has a tip opening too wide for you at the moment. Or perhaps you play too hard a reed. If both of these are satisfactory, daily practice for the next several years will bring about a big change in any case.I am a complete beginner: can play 'Amazing Grace' and some of the notes sound quite nice and saxophony.
The thing is, it plays best at the start and then, in a matter of 2 or 3 minutes, gets much harder to play. Is this telling me something? Great thanks any help.