Have you checked in with your central nervous system (CNS) lately? Years ago, the late Stefan Fernholm showed me this interesting test where you'd take a pencil every morning and put as many dots on a page as you can in ten seconds.
Let's say you knock out 40 to 45 every day for two weeks. Then, one morning, you struggle to hit 30. Now, making dots on a paper is pretty simple, but if you're down 25%, it's bad. For two years, I started my day with a ten-second test. And, after charting all of this, it turned out to be remarkably accurate. When my pencil tap numbers dropped and I continued training, I ended up getting sick and hurt.
The reduced performance on my finger tap test was indicative of CNS fatigue. After that, when I saw my tap numbers drop, I eased my training, increased my protein, and took care of the little things like sleep and resting. It was a miracle.
This "tapping test" shouldn't come as a surprise because our fingers are filled with nervous connections. Some of our most complex movements are the simple ones we take for granted, like typing or picking our nose.
When your CNS starts to feel fatigue, you just can't go heavy any more. Of course, you can always train "medium" (also known as "crappy") for years and years while making no progress. In other words, training a lot at lousy is still lousy. If you can't go heavy, back off on your training until you can!