One thing I have found with my sourdough is that it does not "feed" well on white flour. I don't know if it is because it has been bleached, or what, but it never gets as active, or stays active for so long as if I use whole wheat flour, preferable fresh ground in my home mill.
Yes, adding some fresh (crushed) grapes can help. Grapes have lots of natural yeasts on the skins.
The "sour" part of the sourdough is not from the yeast however, but from the action of the lactobacillus. They excrete lactic acid as they grow. But then as the dough gets more acid, it starts becoming less habitable for them, and more habitable for the yeast. So the yeast starts to take over, producing alcohol as a byproduct (which you can smell in the "hooch" that forms on top). Anyway, the alcohol starts to make the dough less habitable for the yeast, but better for the lactobacillus. And back and forth it goes in a deliciously symbiotic relationship.
Therefore, if you are looking for a real tangy taste to your sourdough, it is important to "set a sponge" for a day or two before you make bread (or pancakes). Just mix a cup or two of starter (I use all but a couple of tablespoons of my starter, and then feed what is left and put it away for the next time)in with your flour and water (and maybe your sugar), and leave it sit at least overnight. You should be able to see it working. Then add the rest of your dough ingredients, let rise, etc. You can get by with no additional yeast, but it is tricky.
Right now I am working on starting a new rye sourdough. My wife thought the last one was some science experiment gone wrong, and she threw it out.