The problem is probably water content of the dough (though it could be a bad yeast culture). Add more flour and knead longer. If kneading by machine, the dough should not stick to the bowl anywhere. If by hand, it should not stick to the heel of your hand as you push into it.
Add about 1/8 teaspoon per 3 cups of flour of vitamin C (ascorbic acid) purchasable from a health food store. Apparently this is done in commercial bakeries. I have so far never tried this.
Reputedly, commercial bakers add diastatic malt to their recipe (it’s sometimes listed in the ingredients. Malt breaks down the starch faster which yields the sugar on which yeast feeds. You might find this from your health food store or order it on-line from bread-making supply store.
You can add honey or sugar to your recipe; Alton Brown does this to about 2 teaspoons per 4 cups of flour (one-pound loaf).
Unless you’ve used too much yeast, this is normal. If you find the smell offensive, you will find it greatly minimized once your bread has cooled.
However, if you’re using the prefermented approach, this should not occur as that approach uses very little yeast.
Salt is necessary to temper the leavening. Salt retards yeast action lengthening the rise, but also moderating it. Figure, according to Alton Brown, 2 teaspoons of salt to 4 cups of flour (about a one-pound loaf).
It is important to catch the sponge (pouliche) as it peaks. Alton Brown’s advice on sponges is to make them ahead of time, overnight in the refrigerator. However, he isn’t preoccupied with catching the sponge at its height. The important thing is not leave the sponge active too long and not to use it too early. My routine depends on when I go to church: if I go in the afternoon, I make bread in the morning, so I leave my sponge out on the countertop all night beginning the bread making shortly after 6; if I go in the morning, I leave my sponge in the refrigerator, but take it out before I go to church, then make bread upon coming home a few hours later.
Other answers include...
This is one of the great variables. Sourdough breadmaking results in a much chewier bread. Nevertheless, chewiness is reputedly a function of the time your bread has to rise before backing—the more structure created by rising, the better the texture. If your yeast or sourdough can survive two rises (few can), then that should improve the texture, but be careful not to mangle the dough when kneading. You can also try making the bread the night before, kneading (knuckling only) after its first rise and storing it protected in the refrigerator overnight. Then let it take its sweet time rising before baking.
This is easy: see “Bread theory” for the what’s going on. You must humidify the inside of your oven during baking.
This comes by experience, but the guidelines are
Sourdough is just a wild version of the same yeast you employ to make “normal” bread, however, it is fully active unlike even “active” dry yeast. Also, because sourdough is grown and fed or maintained in your locale, it is specific to your house, neighborhood or region. No where else in the world has your specific variety of yeast.
As sourdough breadmaking means fully active cultures, your bread rises better, is chewier and more robust throughout the entire process of making and baking.
The yeast you buy in the store, even the best-quality stuff, is mostly dead. When you make bread from it, you’re counting on some of it being alive and kicking off a temporary culture from which you make bread that day. This is one of the reasons Alton Brown and others suggest making a sponge; it’s a way of culturing the store-bought yeast into life, then using it to infect (kick off) the larger quantity of still dead yeast later when the dough is made. To understand this, read my bread recipe.
The sour taste is a function of time in fermentation and rise, yeast culture, repeated fermentation of dough (pâte préfermentée) and down-right dishonesty. The sour-tasting sourdough is mostly a novel creation of San Francisco bakers; I have never tasted sourdough bread in France, for example, that tasted the least bit sour. Remember, sourdough is just a live yeast culture.
I don’t have first hand experience at this, but I understand that San Francisco bakers cheat by adding extra, dead sourdough cultures.
Make certain you aren’t cheating your rise times by making bread at summer temperatures or raising it in a warm place. Let the flour autolyze over time and develop the lactic and other acids that create the flavors take their sweet time.
I have also heard you can use a small percentage of rye flour to increase sour taste. This has not worked for me.
Books such as Baking with Julia suggest a series of fermentations prior to creating your actual sponge (or pouliche).