Nancy Silverton Grape Starter - Does This Look Right?

After a summer and fall of wonderful homemade pizzas using Nancy Silverton's Pizzeria Mozza dough recipe, I have decided to branch out and start baking bread as well. I've been enjoying La Brea Bakery breads for 30 years, so I figured I might as well stick with Nancy for my first sourdough attempts.

I don't have any of her books, but have just ordered several (including the La Brea Bakery Breads book), which are now on the way. I thought I'd get a head start on the sourdough starter, and relied on this recipe/description, which I found online.

After adding cheesecloth-covered, gently crushed grapes to a flour/water slurry, I had good activity between 36-48 hours later. At that point, there was definitely a bit of liquid separation, but there was also unmistakable bubbling action. EDIT: I should also note that the volume roughly doubled during this time, before slowly collapsing back to the volume pictured below.

However, over days 3 and 4 - where I am now - the separation between liquid and flour has become very distinct, with all the flour mixture settling at the bottom and a layer of slightly grape-tinted liquid on top (along with some flecks of grape skin that got through the cheesecloth). The bubbling action seems to have ceased and it smells a bit like sour yogurt. Where the "grape bag" seemed inflated earlier, it now seems deflated.

My concern is about the apparent lack of bubbling. It's seeming kind of inert at this point, which doesn't strike me as correct. One thing where I have read some conflicting advice is about stirring. The online instructions I followed suggest stirring the mixture once or twice per day - which I have done. However, I have subsequently seen some advice to leave the mixture untouched for the first six days. Could stirring it occasionally have caused a problem?

I've also read that in her instructions in the book, Nancy recommends "refreshing" the culture on day 4 (today, for me). Might that help here?

Anyway, if anyone has experience with this starter and has any advice based on the above, I'd love to hear it!