On a new fitness program, I've been steadily building up strength. Lift heavy weights, do complex lifts, eat lots of food, increase weight each session when you ended the last session with good form.
Told, I failed to put 5 kg on my squat, and it was murderously hard, despite last session being not unusually difficult, and not seeming particularly fatigued. I dropped back down to last session's max, and that too was hard.
The first inclination was something like, "What's going wrong with me? What's going on here?"
But that's the wrong way to think.
On a slightly more careful realization, I noticed that,
1) I didn't eat as much earlier before the gym, I just ate a little bit slightly before going, and probably had less nutrients and energy overall,
2) I wasn't as well-hydrated as usual,
3) I didn't take creatine before going, which generally gives you a bit more capacity to work with, and,
4) I didn't take caffeine immediately before going, which helps fight fatigue and have stronger workouts as well.
This is a pretty easy cause and effect relationship to see when lifting weights -- "I'm not fatigued, I'm not ill, and I'm not injured... and yet I'm having a hard time than I did last session, despite the fact that I should be stronger." It's pretty easy to look at how you've managed lifestyle factors around that lifting session, and identify the cause.
Yet, most people do the exact opposite when they're having a bad day in business development, writing, or any other endeavor that takes skill and creativity. People say, "Why can't I do well today? What the hell is wrong with me?"
People discount those lifestyle factors when it comes to creative and professional work... but why? Eating well, being hydrated, rested, and generally doing things the right way has just as much impact on work-life as it does on lifting weights. Maybe moreso.
Next time things are wrong, run through the lifestyle factors that might influence it. Start there. Tune them up quickly, and results improve rapidly.