Could be either or both. If you have weakness in your embouchure, it could be starting out ok, then deteriorating as you play. Blow some long tones, paying careful attention to your embouchure and breath support. Is your embouchure allowing the reed to vibrate freely? Are you biting? Is there tension that causes fatigue? Are you providing adequate abdominal support to your breathing? If there are any such problems with your technique, work on them and see if the problem goes away. The other possibility is that there's a leak somewhere. What condition are the pads in? Are all the mechanisms operating properly? Is there stickiness in the octave mechanism? When did the horn last get any maintenance work? If all else fails, take the horn to a tech who can examine it with a leak light.