Some days, you're just firing on all cylinders, and some days you're not. If I'm not into it on a given day, I can back off on the intensity and focus more on volume, figuring that I can use cumulative stress instead of my typical lets go to war intensity. I've done workouts in the past where I've just picked one exercise (usually a compound), and just done as many sets as I can for 20-25 mins.
After which, I leave and don't obsess about whether it was productive or not. It's like when you have an injury, or even the beginnings of one; you focus on damage control, or maintenance instead of progress. In the long run, it's a good thing.