How long are you fermenting your dough once you've added the starter?

Are you doing a long, cold ferment?  If not... you should try it.

Might be that there's a newer strain of wild yeast that's become dominant in your mother.  If that's so, there's not much you can do for your current mother.  Might start another batch from scratch and see if that helps.

do your first rise in the frige overnight

So, here's how I do it...

When I mix the dough, my recipe mixes all of the water, and a portion of the yeast/flour.  The rest of the dries get mixed, then sprinkled on top of the wet mix, and in the fridge it goes overnight.  The next morning, it gets pulled out, all of it gets mixed together, let it sit for a short bit, then back in the fridge it goes overnight again.  It'll double (or more) during that time.  The third morning, it gets pulled out, beaten down, and allowed to warm and perform the second double.  Then it gets beaten down, formed, and does the third doubling before baking.  

The amount of flavor you get from that is unreal.   Give it a try, and see if it helps with what you're looking for.  I believe that the really good sourdough breads you're thinking of are doing the long, cold ferments like that for flavor.

Speaking of that... it's definitely going to be baking weather still this week.  I should go mix some up.

Try 3-5 days, cold ferment. Then allow a few hours at room temp before using.

I have made some Russian sour black bread. It was quite sour, but delicious.

It used kefir and rye flour for the starter. 50 grams or each approx 2 tablespoons of each. Add each and stir together everyday for 5 days. The rye won't matter when making the bread. The first few days nothing will be happening, then BAM! It will be full and airy on day 3or 4.

Take 2 T aside and use to start the next starter.  Use the remaining in your recipe.

I’ve been following the naturally leavened sourdough recipe from King Arthur.  I get good rise on the dough
I have been making one load either the night I make the dough, or the next day.  The other half up to a week later, as it sits in a container in the fridge.  

Bread has a good sharp sourdough bite.
I let it rise again after gentle kneading in a brotform and then bake in a Dutch oven (about 33 minutes with lid, with some water sprayed in before I put the lid on at 425F.   Then 7 minutes with lid off
Now lately my problem has been the dough collapses right after I take it out of the brotform and doesn’t come up again in the pot.  The pot is round while the form is rectangular.  And maybe that’s part of the reason.   But their other recipe I used  to use (had yeast added) generally stayed rounder.  
Maybe I don’t have enough flour?   It’s hit and miss, and generally the dough that stays refrigerated longer performs worse while baking.   Been thinking of freezing the second portion, or just halving the ingredients


* I found my problem.   When making a new batch of dough today, I realized I missed putting in the small amount of whole wheat flour.   I came out very nice and full today, not flat like it had been the last batch or two.   I guess I got complacent and figured I had the recipe memorized , mostly,   But I did not