A starter that stays flat after a feeding is almost always slow rather than dead. Yeast and bacteria are alive in the jar; something is just holding them back. Work through these causes in order — most flat starters come down to temperature or maturity.
Open the free starter log →This is the number-one cause. Wild yeast is sluggish below about 21°C (70°F) and barely moves near 16°C (60°F). A starter that doubles in 6 hours at 25°C might take 18 hours at 18°C. Move the jar somewhere warmer — on top of the fridge, near (not on) the oven, or in an oven with just the light on. Give it 24 hours at the new spot before judging it.
A brand-new starter often shows a big false rise on day 2 from gas-producing bacteria, then goes quiet for several days while the real yeast establishes. This lull is normal. Keep feeding once or twice daily and a fresh starter usually becomes reliably active around day 7–14.
If you keep adding fresh flour to a large amount of old starter, the yeast cannot out-pace it. Discard down to a small amount and feed at a generous ratio like 1:5:5 (one part starter, five flour, five water). Conversely, if you let it sit two or three days between feeds, it exhausts its food and goes dormant. Feed daily while you rebuild strength.
Heavily chlorinated tap water can suppress the culture — leave water out overnight or use filtered. Bleached white flour ferments slowly; adding 10–25% whole wheat or rye gives the microbes far more to eat and usually wakes a stalled starter within a feeding or two.
Many starters peak and fall back before you check. If it doubled at hour 5 and you looked at hour 9, you saw a deflated jar and assumed nothing happened. Mark the jar level with a rubber band right after feeding so you can see the true peak.
The fastest way to diagnose a flat starter is a feeding record: ratio, flour, room temperature and peak rise time. Once those are written down, the pattern jumps out — and you can watch peak rise times shorten as the culture recovers.
Log feedings & track peak rise — free →