A simple addition of flour and water again today. Plus a bit about wild yeast vs commercial yeast.
If your starter seems a bit thin -add a little flour for a day and see if it evens out. Go at your own pace - actually - the pace of your sourdough - if it doesnt seem active within the first few days - try a warmer place - and just wait a bit - even a thin bowl of flour and water will start to bubble if you leave it on the counter for a few days.
Wild yeast is everywhere, and you can "harness" it out of thin air. Sometimes people use grapes - you know the white "mist" you see on grapes sometimes - that's typically the yeast. Commercial yeast is really only a small portion of the complex and synergistic reactions you get with wild yeast. Real whey is a great inoculate for many cultured processes - (the watery substance that you get from real milk separating into curds and whey). If you don't have real whey, then use the liquid from atop your yogurt ( as long as it contains live cultures).