There are days when our bodies simply don't want to cooperate. That can have all sorts of reasons (sleep, diet, hydration yesterday, how hard that we played yesterday). If we can observe this carefully, maybe some common denominators will pop up. If I drink black tea before playing, my lips get "less flexible". That makes lipslurs and long tones harder to play fluidly. The mystery ingredient is tannic acid found in black tea. A real salty evening the night before also makes my lips feel like rubber. I had a bout with blood pressure a couple of years ago and the pills also changed the hydration of my body and thus my playing.
Additional pressure if we had a real physical day the day before can also be an issue. I noticed this when my kids moved and I helped.
The second half is to stay motivated and in fact I have books of stuff in the "lower" register for days like this: Kopprasch Etudes, Concone Vocalises for alto voice, the good old Lutheran Hymnbook and a couple of books of solo pieces for (junior) high school edited by Don Beeler. The solos at the back of the Arban and Clarke books are also very manageable.