Yes I know it well.
It certainly does pay to be well rehearsed and simplify your playing when you are not feeling inspired. Simplicity is just as effective and it's a lot easier than screwing yourself trying to play at your limit when the mood just ain't available.
Who knows what the magic ingredient is? It certainly is worth putting some thought into your mental attitude.
I play a sport and similarly wonder why I thrashed everyone last week and this week I can't hit the damned thing!
Watch athletes before they compete, the psyching up is of huge importance to their performance, I guess for a musician it's worth indulging in something similar.
Best of luck,
Loxford