Have you been doing your lip flexibility exercises on a regular basis?
So often we tend to say that we have a few "set exercises" that are always included in our daily routine, but when we think back over the past couple of weeks' practice sessions, we may realize that some of those exercises have been practiced rather sporadically (for me, single tonguing is most often ignored). Lip flexibility is another frequently overlooked exercise for may people. The best policy is to schedule these exercises at a specific point of the daily practice routine, usually sometime during the warm-up period.
Below is a progression of three lip slurs which I have found to be very useful, not only for myself, but for my students, as well. You know the routine - go down chromatically through all valve combinations and back up. Those who cannot attain the upper register in a relaxed manner, might find it easier to start at the bottom (1-2-3 valve combination) and work your way up, as high as you can comfortably do so.
I hope you find this to be a very beneficial 20 minutes (18 min. 12 sec. to be exact) in your daily routine. Remember: every day!