Flat and lifeless after a week: it's usually cold. Move it somewhere 75–80°F and feed twice a day for three days.
Smells like acetone or nail polish: it's hungry. Feed twice a day, and consider a larger feeding ratio (1:5:5 instead of 1:1:1).
Gray liquid on top: hooch. Harmless. Pour it off and feed.
Fluffy white or pink mold: throw it out. This is the only symptom that means starting over. Everything else is fixable.
Rises and falls too fast: your kitchen is warm and your starter is thriving. Use it at peak, or refrigerate between feedings.
Not doubling after a week: give it two more weeks. Some starters are late bloomers. Consistency beats intervention.