Hi Alan,
how long have you been playing? it may well be that you are closing one of the stacks (group of keys eg top stack B,A,G bottom stack F,E,D before the other) it could be a slight niggle with your octave mech, or a slight leak somewhere.
If you are relatively new to the sax it may be your technique but to pinpoint the cause get your local repairman to give it a quick look. If you came into my workshop with those symptoms I'd quickly put a leak lioght down to check for leaks and only charge if there was any real work that needed doing.