Couldn't see the forest...

Do you see it?

Let me give you another view:

There's a wide swath of light where there should be a nice checkerboard effect:

Upon closer examination, it turns out it's my old buddy: the red square I thought I had fixed.

Without really looking closely at the problem, and assuming it was just a pair of squares out of place, I had flipped the red and light pair and sewed them back in (digitally represented below).

In actuality, the real issue was the entire 2x5 section. I assembled these 25-blocks by sewing two 2x5 segments to either side of the center 1x5 strip. This one was inadvertently flipped.

This morning/afternoon I'll be picking out, flipping, and resewing this section. I've already done it digitally, below, where you can see that not only does it solve the wide light section, but it also clears up that dark backwards L corner as well.

I'm telling myself I'm glad I'm still able to learn something at my age, even if it's from my continual mistakes!