Ups and Downs in Bread-Baking

I've been trying for about a month to establish a sourdough starter that came in a packet. It's been puzzling me because it never really acted like the starters I've done from scratch. I'll start breads with it, thinking it's active, and end up with dense loaves -- occasionally with massive craters -- scary scary bottomless craters! -- inside:

I did work out, for the bread above, that punching down the dough better might help.

In addition to punching down the dough, I made an effort to feed the starter, carefully, for a couple of feedings, before trying to make bread with it. The latest bread is an improvement (and with a little bit of wheat flour in it, it has a nice, faintly wheaty taste):

Still a few large holes, but the texture is much better, not nearly so dense. By the next loaf I hope to have a nice, airy bread again!