Gary is right, IMO. It takes a long time for the multiple small muscles of the embouchure to learn to coordinate and to gain strength and endurance. UNlike the large muscles that we use when exercising, these small ones, which have a very different vocation, do not give us a lot of feedback on their level of fatigue. In fact, the only reliable feedback is our sound. The moment it starts degrading, it means the embouchure is getting tired. As adults, comebackers or initial learners, we tend to have expectations. We are driven and result oriented, so we tend to be impatient. We devise a practice regimen and it better get us where we want to go, darn it! We often try to do too much too early in an attempt to obtain results. Kids develop very differently, almost in an incidental fashion and can do so for a number of years before getting serious about playing. In my experience, bad days signal overuse of the whole playing apparatus.