Bisquick waffle surface is inconsistent. Likely equipment or recipe problem?

I've been using the same waffle recipe and waffle maker for about 10 years. Lately, my waffles have changed a bit.

Waffle iron takes quite a bit longer to indicate done. Previously I'd expect it to be ready a few seconds after a 5 minute timer went off. Now it can be closer to a minute afterward.
Surface of waffles is no longer as uniform. The center has often either collapsed or otherwise not stuck to the upper iron, leaving an undercooked "divot". (But not 100% of the time. I can still get a good surface occasionally)

It's a 10-year-old Black & Decker that has worked well. The grilles have no obvious damage to the surface, but I can believe that something related to them has changed. I wouldn't be too upset to replace it. (Replacing the removable grilles appears to cost more than replacing the unit). There's only a single ring-shaped heating element in each half. So I don't think the heating is the problem. I'd expect a consistent hot-spot or a consistent cold spot away from the center. Neither of those seem to be happening.

My worry is that replacing would be useless because something is off with the recipe. In my case it's just Bisquick (mainly because I never do any prep the day before, I won't have buttermilk or yeast, and it's worked great previously). This is the only thing I use Bisquick in, and I'm concerned that a change in the product could be the issue instead. My double-batch as made:

576g Bisquick
2 eggs
4 Tbs vegetable oil
2 2/3 cup milk

Each waffle is made with a shy 1-cup batter poured into center.

While I normally try to apply a spray oil before every batch, I do occasionally forget. When that happens, I haven't noticed it being significantly better or worse.

Anyone know if these craters are an indication of a problem with surface of the iron?

While it could be a lack of heat, it looks to me more like the batter in the center is not sticking or otherwise remaining in contact with the upper surface. I'm supposing the grille is hot, but that there's an air gap to the batter. Seems odd that it's always the center where it pulls away.

equipment waffle

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Why are my waffles falling apart in waffle maker?

Waffles might split because the batter is too thin If your batter has too much liquid in relation to dry ingredients, i.e., too much milk, oil, and eggs or not enough flour and sugar, then this can also lead them to split apart when cooking.

What is wrong with my waffle batter?

The problem is moisture. You need some of it to make sure your waffles are tender, but too much and that extra moisture will quickly soften the waffle's outer crust. Buttermilk is also a tricky ingredient because it tends to make a heavier batter, which results in less crispy waffles.

Why is the top of my waffle not cooking?

If the waffle iron is not heated sufficiently, the inside will still be raw when you open up the plates, causing the top and bottom to separate. If your waffle maker has an indicator light to tell you when it is hot, try waiting a few minutes longer before pouring your batter in.

Why are my Bisquick waffles dry?

Don't over-mix the batter.Over-mixing will result in dense, tough, and dry waffles. Use a good waffle iron that will cook your waffles to an even golden-brown color with those beautiful crispy edges.