My Black and Decker electric lawnmower leaves a lot of uncut spots, or
more precisely narrow strips and little round areas. 12 amps. Model
525, the model which preceded the current model 575.

The blade is pretty sharp already, but I am sure I should sharpen it
myself, and I will. I walk pretty slowly. Slower than I would if this
weren't happeneing, but it still happens.

But I wonder if you all know any other reason this would happen?

This is a used mower, but in very good condition. It cuts well, and
mulches well. I have it on mulch. When I was looking at sites that
sold it, one person complained that it left parts uncut. I presume he
had started with a new mower, but maybe not. He said it was easy
enough to get them later with a weedwacker, but there are really a lot
of them and I don't think it is easy. It would be easier to mow over
each section twice.

Probably a mulching blade. I don't know how to tell by looking but I
can describe it if necesary. And it does mulch.

It's not that I miss the area. The uncut area is often right in front
of and right behind areas that are nicely cut, both "in front of" and
"behind" based on the direction I am pushing the mower.

Thanks a lot.